Can he? Will he? Won't he? Maybe

I wonder about the phrase "my master can __ but he won't" what exactly is being said? to say he could __but hasn't as of yet or even that it is unlikely that he would gives a space for the acknowledgement that he might however unlikely it is that he would do whatever that thing is you just never know one day down the line he might decide to do it and it will be ok because it's his prerogative as master.  To me the he could but won't sounds like I can safely say I'd submit only because I know for a fact that I will never have to which defeats the purpose. Maybe it's only a semantics thing but it's one of those phrases that just doesn't sit right.