Depending on what your data retention settings are, it is possible to see different numbers once you move to a 3 month time period. For example, your detailed statistics retention might be set to 30 days. After 30 days, those numbers are summarized into hourly statistics. If your hourly statistics retention time is set to 30 days, your hourly statistics will be summarized into daily statistics. In that case, your 3 month report would include 1 month of daily statistics, 1 month of hourly statistics and 1 month of daily statistics. That would skew the numbers from what you would get if you had run a monthly report at the end of each month before the detailed stats got summarized to hourly stats.
Hopefully that makes sense and is helpful. If not, let me know and I'll try to explain it better!