The issue is that there's no contrast between short-i (as in KIT) and long-ee (as in FLEECE) before ng—that is to say, it's not the case that some words have one and some have the other. That means that in words like "sing" and "mink", the vowel can sort of sit phonetically halfway between short-i and long-ee, and for some people it sounds more like one and for some people it sounds more like the other, and no one really notices because it doesn't make a difference.