AdWords API inconsistency CustomReportJob reports?

Using CustomReportJob, we requested a daily report on Jan 1, 2006 at 11
pm for the same day. Then we requested a daily report on Jan 2, 2006
for the previous day (Jan 1, 2006).

We thought all keywords reported in the daily report collected on Jan
1, 2006 at 11 pm should be included in the daily report collected on
Jan 2, 2006 for the previous day (Jan 1, 2006).

But this was not the case. Some keywords has keyword stats reported in
the Jan 1, 2006 report, but they are not reported in the Jan 2, 2006
report for the previous day.

Why the two reports are inconsistent?