*Just musing to myself here really*
I have been aware that the fibre paper swells when wet (and potentially shrinks when drying), I guess that up to actually measuring the print image size I hadn't followed the logical thought process that with the "grain" of the paper fibres going in one direction the actual swelling/shrinkage may not be totally consistent across a sheet. What I think I mean is that if the centre of a print is the reference point the top/bottom and left/right sides may not expand equally by the same amount.

The follow-on thought is that the image on the paper has been stretched/distorted (very slightly) in one measurement e.g. width in relation to the height. Ho hum - good job I don't have to produce a dimensionally "accurate" image of the subject!