Put the same birth details into two astrology apps and you can get two different charts. Before you decide one of them is broken, it helps to know that most of the disagreement is a choice, not a bug.
House systems
The most common split is the house system. Placidus, whole sign, Koch, and Porphyry each divide the sky differently, so a planet can land in the tenth house in one and the ninth in another. None of them is the single true answer. They are different traditions for drawing the same sky. Placidus is the usual professional default, which is why most reference charts are cast in it.
Where some house systems break
House systems also disagree about what to do far from the equator. Placidus and Koch divide the sky using the time each degree of the zodiac takes to rise and set, and near the Arctic and Antarctic circles some degrees never rise or never set at all. Past that point the math has nothing to divide. Apps handle it differently. Some switch quietly to another system, some refuse, some produce cusps that make no sense. If you were born somewhere like Tromsø or Murmansk, whole sign or equal houses will give you a chart that holds together. Equal houses simply count thirty degrees at a time from the Ascendant, so they never run out of sky.
The zodiac frame
The next split is tropical against sidereal. Tropical ties the signs to the seasons; sidereal, the frame of Vedic astrology, ties them to the constellations, and the two drift roughly a sign apart. A careful app does not relabel tropical text as sidereal and call it done. It writes each tradition in its own terms, because each earned its meanings honestly and reads differently.
Which sidereal?
Sidereal astrology has its own internal fork. The offset between the tropical and sidereal zodiacs, called the ayanamsa, is not agreed to the degree. The Lahiri ayanamsa, the official standard in India, currently sits a little over 24 degrees. Others, like Fagan-Bradley in Western sidereal practice or Raman, differ from it by up to about a degree and a half. A planet near the edge of a sign can change signs depending on which one an app uses. That is a tradition choice too, and a good app names the one it uses.
The same birth, a different clock
Some disagreements are not about method at all. Two apps can read the same birth time and convert it differently, because they rely on different timezone records. Historical timezone data before 1970 is patchy in many places, and an app that assumes modern rules for an older birth can land an hour off. When two charts disagree by a whole sign on the Ascendant and agree on nearly everything else, the clock is the first thing to check. The place is the second. Paris, Texas is not Paris, France, and a place search that picks the wrong one moves the whole chart.
The smaller forks, and the part that is not a choice
There are quieter forks too, like whether an app uses the true lunar node or the mean node. All of that is method, and reasonable practitioners differ. What is not a matter of taste is the astronomy underneath. Whichever system you pick, the planet has to be in the right place first. Disagreement about method is healthy. Disagreement about where Mars actually was is an error wearing a method's clothes; telling the honest choices from the real errors is the whole of what an accurate astrology app has to get right.
Common questions
Which house system is the most accurate? None of them, in the way accuracy usually means. House systems are different traditions for dividing the same sky, not competing measurements of it. Placidus is the most common in professional practice, and whole sign is the system the Hellenistic astrologers used. Pick the one whose reasoning makes sense to you, and read consistently in it.
Why is my sun sign different in Vedic astrology? Because Vedic astrology uses the sidereal zodiac, which currently sits a little over 24 degrees behind the tropical one. For most people the Sun moves back one sign. If your Sun sits late in its sign, it may stay where it is. Neither answer is wrong. They are different frames.
Why do two sidereal apps disagree with each other? Usually because they use different ayanamsas. Depending on which offset an app chooses, the sidereal zodiac can sit up to about a degree and a half further along, which is enough to move a planet near the edge of a sign. Check which ayanamsa each app names before comparing the charts.
Should two astrology apps give the same chart? For the planets, yes, to within a small fraction of a degree, once they share the same time, place and zodiac. For the houses, only if they also share a house system. If the planets themselves disagree by more than a degree, one of the apps has an error, not a method.
Celestium lets you choose your method instead of locking you into one. Placidus by default, switchable to whole sign, Porphyry, or Koch, with tropical and sidereal each written natively and the true node used throughout. Under all of it, the positions are verified against the professional reference standard, so the choice stays yours and the math stays honest. See how on our Methodology and Accuracy page. Celestium is free on the App Store for iPhone, iPad, and Mac.