It's imporntant to note, that just because you're using broad matching, doesn't mean you're actually broadmatching words:
[webmasterworld.com...] msg 19
Shiznaught, also please see msg #8 in this thread for other details about broad (and phrase) match:
[webmasterworld.com...]
If I had to sum this topic up in one and a half sentences, it would be as follows - quoted from the post mentioned above:
...the essential intent (and function) of broad and phrase matching is to show your ads for as many variations as possible, so long as those variations deliver relevant ads to Google users.However, broad and phrase match are not meant to be a guarantee that one will appear for any and all variations of a keyword, regardless of whether or not they'd deliver relevant ads to Google users.
Hope this'll help.
AWA