Thackeray in his Septuagint Grammar (the very best there is...eisenbrauns sells it as a reprint for $77) writes a fairly good bit about these kinds of words. They fall under the more general catagory of "Metaplasms.
H.St.J. THackeray wrote:We may group under this general head further instances of the mixture of forms and declensions which grammarians subdivide into (a) abundantia, viz. double forms for nominative and other cases, e.g. lew/s, lao/s : (b) heteroclitia, viz. a single nom. form with diverging forms in the oblique cases, e.g. os and ton sko/tos : (c) metaplasta, viz. formation of a new nom. out of the oblique cases, e.g. hH wdi/n. Mixture of this kind was common in the koinhv and has already been illustrated in the preceding sections : several of the instances which follow have classical precedent.
It is strange, but it does happen to a number of different words. They just change gender. It is indeed a sign that you are very observant though. When I tend to read, I'm usually moving through at a pretty good pace (unless I'm diagramming) and I miss that kind of stuff.