Are you measuring the linen?
Can linen backing change the dimensions slightly? i.e. the paper gets a bit 'stretched'? I thought I remember reading that once, but may have just imagined it.
I think it’s not uncommon for posters to be wider or smaller by a 1/8th, 1/4, or even 3/8th of an inch, on the longer side. However half an inch on the width is pretty uncommon for sure.
On the other hand, T, there is the completely "up front" Sophia Loren...Okie