There are a few reasons why this may happen. The timeframe on a notification email is an estimate of when sales are received. Orders placed near the beginning or end of that time frame may arrive in a notification batch that is prior to, or subsequent to, the timeframe on the notification email in question.

If there is a large difference in the time scale, the sale in question was probably placed in an ON HOLD status previously and we received additional information during the time frame indicated in the email.

Alternatively, you may want to consider time zone differences for the discrepancy.