After reading a long thread I've read that Seraph Weapons "power up" from their initial stats. Some suggested that killing enemies raised their stats or damage, and a great deal of people seemed completely unable to reproduce this.
So do Seraph weapons really power up, and if so, how? Here's the thread where the rumors/hints were suggested: Seraph weapons secrets. It sounds like trolling considering the lack of confirmation reports, but I don't have a weapon of my own to test.