I believe it is because of the machine that folds them. there are 2 machines, a printer and a folder/stuffer/sealer
when it gets out the of printer, you move the stack of papers to the folder machine. you have to flip them a certain way to get them to fold right, if you dont they fold backwards.
I did newletters for a non profit once and they they had a machine that did the folding..it is a pita to go through all the papers to switch them to the right way after they came from the printers, so we just left them that way and they folded backwards.