There's weird gaps where groups have some posts from a given date but not all of the posts (e.g. I know I posted to csm.hypercard on 24 April 1990 and can see i

There's weird gaps where groups have some posts from a given date but not all of the posts (e.g. I know I posted to csm.hypercard on 24 April 1990 and can see it in Google's dejanews cache, but not in this cache which has other posts to the group on that date). Was it just luck of the draw from what was cut to tape? How do you handle cross posting?