Just a guess, but it's probably because the finger-joint boards are made from short offcuts from the manufacture of larger boards like 2x4s joined together with

Just a guess, but it's probably because the finger-joint boards are made from short offcuts from the manufacture of larger boards like 2x4s joined together with long zigzag joints. Saw kerfs and finishing sanding probably account for the ⅛" difference in size. It's usually not a problem, except when it's used beside a 2x4 on the flat, in which case the 2x4 needs to be thinned down on the table saw.