thats a fair point, theres possibly more of these 'mutants' generally but a very careful, observant breeder will notice them more often.This may very well be a developmental anomaly. However, inbreeding CAN lead to developmental abnormalities as well, showing up in increased frequency the further down a generation is inbred. This has been shown to be true with D. melanogaster, X. laevis (both popular models for study) as well as with plants and birds, among others. Inbreeding may not be a factor in this case, but that line of thinking is not entirely off-base.
It should also be noted that with animals that produce a large number of offspring (i.e. Tarantulas), the possibility exists of a random oddity being produced. The probability of that happening with two genetically distinct individuals, however, drops, but the possibility is still there. Separating the eggs from a sac to ensure a high survival rate provides evidence of this being true. If these oddities were allowed to develop in the sac, they may have been eaten by the other sac-mates.
Good luck with those two, er, three Rob, and keep us updated!