Statham was supposed to be in his normal accent for the film and was for most of it, it was just that when speaking to De Niro (the only real American actor in it) he occasionally slipped into mimicking his accent
My next door neighbour has an American wife, and you can hear when he or other friends or relatives speak to her they occasionally say things with the same twang. You even get this across local dialects.
Talking of accents - Russell Crowe as Robin Hood