Yep, no problem gently stretching frame a little here cold , just check both sides are even centrally. I have used a small car jack on many occasions, if one side is off centre, and one side straight, clamp straight side and frame seat post to a bench to keep it stable, and gently push the other side out.
Probably happened when someone fitted a smaller wheel at some point.
As an aside, the rear end is very easily distorted laterally if the wheel is not in it, so when shipping a repaired frame, always pack it with a piece of tube the correct width, bolted in snugly with a threaded rod thru the wheel mount lugs. Not only prevents shippers from crushing the frame, but acts as a handy carrying hold :)
