The forms Isabella and Anabella are Italianizations of names ending in -bel (the Spanish Isabel and the English Annabel) by analogy with the Italian bella, "beautiful" (fem.).
In fact, something similar happened in French, where Isabel was transformed into Isabelle, because -bel was too symilar to the word bel, "beautiful" (masc.), inappropiate for a feminine name, and replaced by belle, "beautiful" (fem.).
The Spanish Anabel has nothing to do with the English Annabel or the Italian Annabella, because it comes from AnaIsabel. Since it is very similar to those names, some people uses it as Spanish equivalent of them, but this is not the etymology of the name.