Do I hear a "what's the difference?" ? 

Think about it. 

A guy (or a girl for that matter) is going to write an exam. Tell him 'Best of Luck' and it implies "you'd need all the luck that you can get to get through this one. I don't think you are good enough on your own to get through with it."

Tell him "All the Best" and you are just wishing that he turns out fine in all respects which implies that "I believe that you are good enough to get through this with your capabilities and may you fare well through this and through the path further ahead".

Most people don't realize the very subtle and yet significant Difference between the two and, frankly, most don't care. But, it becomes significant when you say this to someone who realizes it. And, of course, the other way round works too. When someone who realizes this wishes others with these wishes, listen carefully and you'll know what he thinks about them.

Got a problem with this theory of mine. Then bring it on! ;-) 

Regards

Diff.Thinkr