Since ther is more competition for the keyword widget then for widget extra plus 2000. The bids are higher and so is the payout per click with adsense.
There is software out there made to do just this. There are guys that make their living doing this crap.
Google thinks they can write a script that will catch these things, but they'll never get them all.
And don't forget Google not only gets paid a low cpc for an obscure word but also a high cpc for an general keyword