Ummm, I am a carpenter and I think you will find that all the 2 x4 lumber is 1.5" by 3.5", Every 2x4 has been like this since the early 1900s. It gets weirder when you get to a 2x8 which is 7.25" and a 2x12 which is 11.125". The 1.5" dimension is very standard in north America and all the framing squares and many tools have 1.5" widths for this reason.
 
Mike