Out of order events

I look at the "Snapshot and Archive Stastics" tab in SMT everyday. The count was ~550 (for archive) and kept increasing by couple of tags everyday. I checked the tab this morning and the count showed 20.

I also see "Out of order snapshot events" and I am not sure how it is different from "out of order events" (archive).

Any assistance is greatly appreciated.

Thanks,

Arti