What's the difference between FFKM and PTFE?
Teflon (PTFE) and Kalrez (FFKM) are different polymers. 

PTFE is made from tetrafluoroethylene, and contains only carbon and fluor. 
The material is stiff. 

FFKM is made from tetrafluoroethylene, vinylidene fluoride and perfluoromethylvinylether, so besides carbon and fluor, it also contains hydrogen and oxygen. 
These additional monomers make the polymer an elastomer.