Maybe because people voting know that the Browns did more than just acquire Stallworth. They also got Corey Williams, amongst others. I also believe that you have a lot of Browns' fans voting to help their cause.
The poll is a little misleading. It asks which team has had the best free agency, then puts a specific player next to the team. The Browns did more than adding Stallworth, and probably better moves than just adding Stallworth.
Besides, I think the trade for Simoneau was actually a good move at the time. I think most people liked the Sean Payton house cleaning.