If you have used another calculator and got a different number, this is almost always why: there are two schools of adding up a birth date, and they disagree. Most free calculators use one of them. Zodiacs defaults to the other.
Neither is a bug. They are different counting conventions.
You can switch at any time under Settings → How we count. The page shows both results side by side with the arithmetic spelled out, so you can see exactly where they diverge before you choose. Your calendar re-marks to match whichever you pick.
Compare both schools