Can anyone tell me what sets the next EPG scheduled call date/time?

I had assumed it was set via the prior EPG data, but I have noticed in last week or so that the scheduled calls are becoming more frequent.

For example tonight my TiVo is reporting (via Tivoweb/Phone):

Last Successful Call: Tuesday, Feb 21 at 6:56 pm
Last Call Attempt: Tuesday, Feb 21 at 6:56 pm
Last Attempt Status: Succeeded
Next Scheduled Call: Tuesday, Feb 21 at 8:22 pm
Prog. Guide Data to: Tuesday, Mar 07, 2006
Software Version: 3.0-01-1-000

Anybody have any ideas?