Posted 26 January 2014 - 16:20
There are small definition update packages that are released multiple times a day I believe. There's a button or something you can click on to see which packages were updated. Updates in general are released as and when ready. The auto-update feature will check periodically throughout the day. At the times you're manually triggering a check, you just so happen to be doing so before the next scheduled auto-update.
I really wouldn't worry about it, just leave it to do its job like I do.