"Master" - Has high rank, higher ego. Likes the idea of having everyone refer to him as "Master", secretly wants a senior student named "Igor".
"Grand Master" - With all these masters floating around needs a way to distinguish himself as superior to them, it also sounds somewhat more sophisticated as its usually used to refer to really smart guys that play chess.
"Professor" - Is above the idea of being refered to as "Master" as that is silly kung fu movie nonsense, but is still uber-good and wants people to know it, so uses a title that is usually reserved for really smart people with PHds, afterall, he's smarter then all those Phd's when it comes to martial arts. THis title also implies that his skill is intellectual, not physical, so no worries about having to actually do anything physical... like a situp.
"Soke" - All those other titiles are just not good enough to showcase his great powers of awsomeness, a new one is needed that sounds really Japanese and traditional, one that can only be inherited through families going back hundreds of years... thats the sort of title this person prefers, but he can't get it, so he just claims it anyways. Not his fault those Japanese people won't recognize him, they are just jealous of his super-skills anyways.
Hope that clears thigns up