I don't get it :o(
One common cause, though, would be that the keywords with zero impressions might actually be duplicates of other keywords in your account.
For example if you had the following three keywords in your account as broad matches, probably only one would get impressions - since in the eyes of the system they're all actually the same keyword.
chrome fuzzy dice
dice fuzzy chrome
fuzzy chrome dice
Another possibility is that the zero impression keywords are somehow 'sensitive' in nature, and won't show your ad until they have been reviewed and approved.
AWA
Thanks, AdWordsAdvisor, but ALL my KWs are phrases and they've all been in my account for weeks or longer - plenty of time for G to complain if they didn't like 'em.
Oh. I guess you mean "phrase matched", or enclosed in quote marks. I was thinking you meant phrases as opposed to single-word keywords.
Language! What ya gonna do with it!?
Well, in that case, I guess I'll fall back on my old-standby advice, which would be to contact AdWords support, as they can see the actual account - and can offer you an informed answer.
BTW, what does the 'Status' column of your stats say for the zero impression keywords?
AWA