Well, yes, I am working under the assumption that if there is only one adult in a certain caste, then the young from that caste will be descended from that singular adult. I shouldn't think that's a super weird assumption.

This is a pretty classic case where if you're asking for the canon answer then it's pretty clear the Condesce and her descendant are always the two fuchsiabloods and therefore they always have the same lusus; but if you're asking what you could do for a fantroll then yes you could very easily come up with an excuse for an outlier existing.