Several technical reasons can cause discrepancies:
1. Solar term boundary handling. The most common error. If a birth is near a solar term change (e.g., near February 4th), a miscalculating engine may assign the wrong Month Pillar.
2. True Solar Time correction. Some sites don't apply it; others apply it differently. The same clock time in different longitudes produces different charts.
3. Day Stem calculation method. There are different mathematical formulas for calculating the Day Stem. Reputable sites use the standard method; some don't.
4. Luck Cycle starting age. Different formulas exist for calculating when the first Luck Cycle begins. Slight variations are normal; large ones signal an error.
5. Timezone handling. Misconfigured timezone conversion can shift the birth hour.
Always use a trusted source that explains its methodology. Our engine follows the classical Zi Ping standard with True Solar Time correction.