I'm sure the sizes made sense when a 1 x 10 for instance was actually 9-3/4 to 10 inches back in the 'early days'
I've got a 130-year old house with:
'real' 2 x 4's (130 year old Yellow Pine, eats saw blades and drill bits)
1-7/8 x 3-7/8 '2x4's'
1-3/4 x 3-3/4 '2x4's'
1-5/8 x 3-5/8 '2x4's'
1-1/2 x 3-1/2 '2x4's'
my scrap pile is well stocked with 1/4, 3/8, and 1/2 inch stock for shimming to get things level.....