Netflow analyzer question

Hello!

In the beginning of this month I analyze statistics for last month. Statistics I do daily and for all month. When I have counted daily statistics with that which was displayed for the whole month there was a discrepancy. Mistakes were in days (but not in all days) on borders or a number of the periods granularity. In some days, when the period granularity became identical to all last month (more than 21 days, but less than last 90 days) the statistics on problem days HAS CHANGED also total daily statistics began to coincide with the general statistics for a month.

Can you Explain please as can be, that the statistics for the certain period of time (in my case statistics for one day) varied depending on time of the past from the date of gathering this statistics?

Please if i did explain something confusing tell me and i will try to clear the situation more. I just want to understand why it happend and what the cause of that situation.

Thanks a lot in advance!!!