It’s not the wrong prefix. It’s the same ‘un’ you get in e.g. ‘unlock’ or ‘untie’. The ambiguity of words like ‘unlockable’ or ‘untieable’ is a standard exercise for morphological tree drawing in Linguistics 101 ([un [lock able]] or [[un lock] able]). The existence of structural ambiguities in words isn’t really any more disturbing or problematic than the existence of structural ambiguities in sentences (e.g. ‘John saw the man with a telescope’).

(Technically, ‘unlockable’, ‘uninstallable’ etc. don’t exemplify pure structural ambiguities because the ‘un’ prefix is used in two different senses, but the two meanings do have different structures.)