"Normal" lemons are mutants. They're a hybrid of two different Citrus species, citron and bitter orange, that both look weirder than lemons. In fact, one variety of citron, Citrus medica var. sarcodactylis, is nicknamed "Buddha's hand" and looks like this:
So what it appears (to me, at least, as someone who isn't a biologist or citrus expert) is that that lemon happened to mutate again in such a way that it started partially expressing an ancestral form.