[Brightcolours]

I suppose that they slightly longer focal length is a trick that gives a shallower depth of field at a given f/stop; compared to a 50mm lens it's about 1/3 (?) of a stop shallower DOF.

 

I know that my 58mm f/1.4 Voigtlander Nokton has the same DOF wide open as a 50mm f/1.2 lens.

 

So, with a 55mm (or 58mm) lens, it should have a bit more blur at the same f-stop than a 50mm lens.

I think that mostly the choice has to do with that 55mm gives a bit more room from the (probably f-) mount, making the design possible in the first place.