High tension causes racket frame to change?

I owned a FELET racket called TJ 1000 Power, I notice that the racket’s head tends to deformed when stringed even only at 26lbs, based on the spec the racket can hold up to 35lbs. Without string the racket has like an isometric frame but once it’s stringed the racket become almost like an oval shape. Is this due to the method of stringing or frame issue?