I've only encountered it as a Scottish variant of Annabel. If you don't emphasise the -bel in Annabel (it usually gets a secondary stress, and the main stress falls on the first syllable, the Ann-) then you can easily hear it as Annab'l, and from there to Annap'l is no great distance. As for the single n instead of the usual double, that makes no difference to the pronunciation so it's nor a surprising change. Annabel is originally a Scottish name, based on some long-ago person's misreading of Amabel, so there's your connection!
Very interesting to see it turn up as a surname. Thank you for that.
All the best