- tease coming ...
I believe in the Bxc4 games, the player of the White pieces actually slammed the bishop down on the c4 square in a violent motion, grabbing the pawn in the process. In the games with Bc4, a gentler motion was used. This latter variation is not considered to be as aggressive a continuation as the former.
If you go through this line in explorer:
1. d4 d5
2. c4 e5
3. e3 dxc4
4. (Both Bxc4 and Bc4 are listed, and have slightly different percentages though they are obviously the same move)
I'm not sure if this occurrs elsewhere, but thought I would let you guys know about it.