Difference in blades for 52" deck.

Got a 1500 z Ferris and it lists two blade numbers. I've done a lot of searching and wonder why the same blades have two part numbers. The only change I can see is the cheaper blades are .204 thick versus .250 for the better blades. Both part numbers are called hi-lift and have the same dimensions with the pictures also being the same. The 5101756 and the 5021227 seem virtually the same.

Any ideas?

Difference in blades for 52" deck.