Four honest sources of difference
First: the Si Hua table. The Flying Star (全集) and Three Harmonies (全书) tables disagree on three of ten stems — Geng swaps Ke and Ji entirely. Second: the month-stem method (Kwok Man-Ho) reads transformations from the month stem instead of the year stem. Third: the late Zi hour — 23:00–24:00 belongs to tomorrow in most schools, today in others. Fourth: leap months, assigned whole or split at the fifteenth day.
Notice what these have in common: none of them is a bug. They are documented, centuries-old school differences. The chart is only 'wrong' when the site won't tell you which convention it used.
A checklist before comparing readings
Before you argue about accuracy, align the settings: which Si Hua table, which stem, which Zi-hour rule, which leap-month rule, which timezone convention. Five toggles decide the whole chart.
Our answer
FateZiwei exposes all five: Flying Star / Three Harmonies / month-stem Si Hua tables, Zi-hour assignment, leap-month handling, and timezone — switchable on the chart page, with the active convention always visible. When two traditions disagree, the honest product shows both, labeled.