This can happen when the time range you specified in the time report criteria splits a weekly timesheet.
For instance, let’s consider the following situation. The “From” date you specified in the time report criteria is Wednesday March 14th 2012, and your Week Management option is set to “Monday to Sunday”. If a user has recorded time against a task only on Monday March 12th and Tuesday March 13th but no time on that task for the remainder of the week, the AceProject time report will show a time item for that week with 0 hours on it (because the time that was recorded on that task is outside the time range specified in the time report criteria).
Similarly, the number of hours on a time item in an AceProject time report may be lower than the total number of hours on that time item if time was recorded on days that are outside the date range specified in the time report criteria.