
I can't see your thinking at all.
7 pups is not 'only' it's a good size litter. the other that you spoke of had small litters as did her Dam, not unexpected at all, and could be for any manner of reasons passed down.
Maybe their hormone levels are a little out etc.
Bitches seasons vary, the hormone levels can vary as can the presence of infectious or other agents affecting conception or pregnancy.
I have had bitches miss now 5 times, each bitch getting in whelp on the next attempt (though waiting to see if this happens this time to the girl who missed last time), mated at the same times, in each case I used a different stud than the previous attempt, but in most of the cases the studs produced pup to other bitches after.
I have had between four and 9 live births though have reared from 3 to 9, and most often have had litters of four or 7 pups, interestingly from the same bitches, in one case from the same pairing 4 first time and 7 the next almost two years later.
Barbara and the Grey Curly Tails.