For the same reasons, Gremlins can be better than Master Gremlins.
Also, a Green/Red dragon can turn out to be stronger than a Gold/Black dragon if there is the Resurrection magic... :rolleyes: but this is, of course, a rare occurrence.
In some situations, it is not beneficial to upgrade skeletons, as they will be restored to 2/3 of the number they would have been if they were simple skeletons.