spelt is mentioned second, where-as spelled is mention first.
In all dictionaries, the first is the first form that had come about and the second is the result of either slang or a slight change in language due to location or the dialect of a region.
Therefore, it is actually spelled. but spelt is no longer incorrect, as so many people use it.
It's much like the pronunciation of often. You're not supposed to sound the t, but it is now acceptable to, as so many do it.