OK, let's me explain my thought. I am South. The first two bid of mine is forced, so when my partner jump to 6♣, I think my partner think that we can make 6♣ without the trump K. Now I have the trump K plus the J (the singleton spade is also a push factor), so I bid 7. IMO, I think North should bid 5♣ but not 6, his two forcing bid has already shown his value.