A Raised Question

In reference to a recent post about my personal speech patterns regarding the words when and went, I commented that I produced these with the vowel [ɪ]. I thought that it had something to do with the pattern of w_n, and maybe it does, but it was recently pointed out to me that I have another alternate pronunciation. The part of the conversation went something like this:

I'll just have to talk to them.
[aɫ.ʤʌst.hæf.tə.tɔk.tʌ.ðɪm]

Them?
[ðɛːm]

This leads me to believe that maybe there is some other reasoning behind it. It comes to some problems, though, since the only similarity is that they both precede a nasal sound, and maybe that they follow a voiced sound, but I don't think that's the case. Even the words whence and then which have very similar contexts, but both have the [ɛ] and not the [ɪ], so it doesn't even look like an automatic change. The only thing I can think of that groups these words is that (1) these words are both quite frequently used "basic words" and (2) not likely to be confused with similar words with a higher vowel, such as then and possibly thin (or whence and wince, but rule one takes care of that). Another possible rule or set of rules is (3) certain phonetic constraints must occur, such as voiced_ or _nasal. I can only say that these might be possible reasons, and I really cannot conclude anything past these as to the mechanism(s) for why these words were chosen. One thing though, about the [ɛ] to [ɪ] switch, is that the latter is relatively shorter vowel, and requires less articulation. It's possible that a large part of this change is merely in order to ease pronunciation, I do also tend to like higher vowels (such as my substituion of [eʲ] for [æ] in front of [ŋ]). I'm going to start keeping track of similar contexts and words and see if there is any more of a pattern.