cab not lining up with cab frame mounts

[noob]

Sorry for the endless stupid questions but my wife and I bought this truck in pieces and we are trying to put it back together. we put the cab back on the frame and tried to push the cab as far forward as possible (due to a fender fitment problem). I can bolt the cab down through all 4 corners, but the passenger side rear cab mount sits a 1/2 inch to the rear of the cab, whereas, the driver side rear cab mount sits flush with the cab. The cab is straight according to being flush with the two front mounts. I'm just curious if anyone has had this problem? Is it a big deal? or is it just because these trucks were put together like crap from the factory? Thanks for any help.