This is a great point. Our tool does a lot with long tail and generates a really comprehensive keyword list (one of our big features) so we've definitely seen this a lot.

Are you bidding MORE aggresively on tail terms than broader terms? Google will enter your two keywords into an auction and choose based not just on Quality Score but also on bid (the same way they would calculate ad rank if you were bidding against someone else).

An alternative means of dealing with this is to exact match mans pants. The problem here is obviously that you miss out on a lot of variations you might not know about, SO what you might consider doing is something like this:

• Mans Pants (Exact) - 5.00
• Mans Pants (Phrase) - 1.00
• Green Mans Pants (Phrase) - 5.00
• Red Mans Pants (Phrase) - 5.00

 Bit crude obviously but the point is that if you can massage match types and bids enough some times you can effectively manipulate things and gain some control. Brad Geddes had a great post recently that I think is interesting and related on why your broad match keywords aren't really the ones converting.

Tom