How des Gap Sizing come up?

[Chertsey]

It say s = 8=10 and M = 12=14

if I'm 10-12, what size do In need? Never bought an 8 or a 14 in my life!

Very hit and miss. Some items are huge.

Hit and miss, but most are quite generous so I would assume on the large side, so go for the S if you are a 10 - 12.

Massive. HTH
They come up at least 1-2 sizes bigger than the label in my experience.