I really should know the answer to this but I don't.
Take a piece of sawn timber...quoted at 50mm x 100mm ..is it really that size ? What's the largest cross-sectional area that could be planed out of such a piece ? If it really was 50mm x 100mm then I guess I'd expect to get 46mm x 96mm maybe ? But if it was actually, say, 48mm x 98mm then it wouldn't.
I see that Travis Perkins offer a piece 47mm x 50mm so what's all that about ?