I had been browsing over on myArmoury and came across
this thread which in turned led to
an earlier related thread but that also included a link to several images of a
very similar Norwegian bondeøks to the one at the top of this thread.
I contacted the Maihaugen museum as to whether there might also be such curvature on their axe, as it looked like it might have come from the same workshop and was delighted to receive a reply that the curvature of the hafts (in the 'third' dimension) is indeed an intentional design feature sometimes seen among the type A bondeøks (the type most reminiscent of the 'Viking' style!)