Won't say it isn't a possibility, but I'm leaning toward the simpler answer. Rivulatus, just not the best specimen, and one a conscientious breeder would have culled-- or at least not sold commercially. Doesn't make it cosmically wrong to think it's cute and keep one of them, it's just bad business not to care more about the quality of what you sell. But volume breeders supplying large distributors or big box stores aren't always looking that much at the individual fish they sell, it's more about numbers.
From even non-related good quality pairs you can get the occasional misfit out of hundreds (or thousands) of fry-- in fact, do enough breeding and you probably will, and then it's up to you what to do with them (lot of them will be weaker and won't make it past an inch or so, anyway). Doesn't mean yours can't live a reasonably long and healthy life, unless there are other issues along with the cosmetic ones. Some cosmetic issues don't go much deeper and the fish is otherwise pretty healthy, but sometimes not, sometimes it's just a weaker fish.