are you sure you measured them correctly? you focused at infinity and then used the rear nodal point which is not often even marked any more stuck a micrometer into the lens through the rear element?
I used to measure the sizes of objects created at infinity with a collimator, kind of a variant of 'the hard way' described here -
http://www.bobatkins.com/photography/technical/measuring_focal_length.htmland yes, very few 50mm were 50mm. A few macros actually were, most others were 43-44mm.
