In multilayer pipes, why is there a 25mm and a 26mm?

[lotstodo]

Hi. I'm plumbing an old house in Spain (Among lots of other work!). I'm going to use multilayer pipe of varying sizes but when looking there is a 25mm and a 26mm. Can anyone tell me why?

 

[ShaunCorbs]

Different manufacturers so one started doing saying look ours has extra flow potential now a few offer 26mm

you won’t notice anything so pick what’s easiest local to get / been around locally for a while