Slitherine

Have not played yet but from reading the rules there does not seem to be much difference in manouvreability between mounted and foot.

What is the reasoning behind that?

Mounted are far more manoeuvrable thanks to the ability to turn 90 and move (if they pass a CMT). The ability to do a 3rd move when away from the enemy also helps.

Play some games and try it and see.

Nik is right, having been on the receiving end of a lesson in mounted movement from Alisdair I can testify that the mounted REALLY do manouvre

Mounted being able to move 15" versus 6" for infantry is considerable.