(UK) Timber stud sizes and types, 38mm vs 50mm
Timber is dimensioned in 'nominal width' which means the size it is sawn to when it's processed. Studs are finished by planing, which reduces the nominal width to the actual physical width you'd measure when you installed it.

This webstore's description makes a bit clearer. It specifies that a nominal 50x75mm stud is reduced to 38mmx63mm by planing to produce a smooth surface.