The vaguaries of mass manufacturing.
Eldest cub bought a pair of trail shoes recently and didn't get on with them as ehy were a bit flappy for him. I asked him what size they are and they are my usual size. He's just dropped them off and guess what? They are the very same brand and model that I bought recently but one was too tight around the area where the toes meet the rest of the foot (I'm sure it has a proper name but hey, I'm not a medic!).
Anyway, I try them on, left foot (bigger foot) first. That pretty much fits as on the pair I rejected so I thought hmmm, they'll be the same. However, right foot also fits nicely. How utterly strange.
Examining the shoes against my old Asics 2070's I note that the lace section is shorter on the new Asics Somono 3's and thus the toe box is deeper. This I think is the crucial difference causing the variation and ultimately the issue. If I can be arsed I'll write to Asics but I guess that they'll not bother with an old whinging goat like me.