Batch processing (time) to accurate?

I have a cron job that runs every monday at 8:00 / 8:15 / 8:30 / 8:45
I have jmailalert configured with batch size 100 (mail limit for provider)
I have in total (for this example) 280 users

Week 1:
8:00 > 100 mails sent
8:15 > next 100 mails sent
8: 30 > remaining 80 mails sent
8:45 > 0 mails sent

So far so good

Week 2:
8:00 > no mail sent
8:15 > 100 mails sent
8:30 > next 100 mails sent
8:45 > remaining 80 mails sent

Week 3:
8:00 > no mail sent
8:15 > no mail sent
8:30 > 100 mails sent
8:45 > next 100 mails sent

See the problem? I think the time logic is to accurate (second?): mail for a particular user is sent at eg 08:15:02 (database: _jma_subscribers > date > 2013-11-11 08:15:02) so the next time the badge runs at 08:15 it doesn't sent this users mail because the frequency of 1 week has not past (2 seconds).

Workaround is to weekly sync all users with a date one week in the past.

Hope this makes sense if not feel free to contact me.
regards,
Ruud.