I noticed that on our GFCI breaker, which was a Square D pro (Not Homeline) series. There was about a $20 price difference between the 50 and 60 amp GFI breaker, with the 60 amp being the cheaper one.
Puzzled by this, I compared them side by side and the only difference was that the 50 amp had a terminal for the neutral, and the 60 did not.
Our spa does not require the neutral, as it runs the pumps and heater on the 220 and the rest off of a transformer, eliminating the need for the 120v leg provided by the neutral.
Does your spa require a 60 amp breaker and a neutral wire? If so, I'd ask them to suggest the brand/model of breaker they use.
Using a 50 amp breaker instead won't really hurt anything, but you may trip it if you pull full power thru your wiring.
Brewman