Shimano vs SRAM 10 speed casette

Hi

I was about to change my Shimano 10 speed casette (CS-5600, 12-27) with a similar SRAM (PG-1050, 12-25), but it does not fit. The smallest cog doesnt grip the freehub splines. The freehub seems 1-2 mm too short.

They should be interchangeable, right?
I've used Shimano and SRAM casettes interchangeably on my 8-speed commuter bike and never had any problems.

Any obvious mistakes I could have made?