In the old days, saw mills cut logs into "rough" lumber, wherein a "2x4" board was actually 2" tall and 4" wide and not very pretty. Later on, after it had dried out a bit (and usually done a bit of twisting) during transport and storage at the lumber yard, many customers wanted it "dressed" so about 1/4" was planed off of each face to make them clean and smooth. Thus the nominal "2x4" was whittled down to an actual 1-1/2" by 3-1/2" that we see today.
Larger timbers lost even more, today a "2x10" is only 9-1/4" wide. And since "1x" lumber is thinner they only planed off 1/8" from each side and actual thickness is 3/4"
Obviously, that was back in the old days. Today lumber is only cut once so those nominal sizes really just give sawmills a bonus.
Generally in the US if you request "rough cut" lumber you do get the full size, but that is uncommon in general building projects. It is more common in specialty construction such as outdoor pergolas or fences.