Check out this link if you want to know why this happens:
http://www.operationsports.com/forum...e-answers.html
The reason Dumervil makes such a good DE is that pass rushing OLBs in general often have ratings that are more DE ratings than OLB ratings. As you can see in the above link, FMV and PMV are much more important to a DE's overall rating than an OLB's overall rating.
In my franchise I have a rule about position changes for stat increases. In season 1 I don't do it at all. After season I will do it but only in the offseason (which is when you see this happen in real life). I also only do the switch if the player's size and stats look like they could play in their new position. That means no small DE/DT's and no huge FS/SS's.