What could be the reason for this RS422 wiring?
Most folk wiring up differential serial interfaces just assume the there are two TX wires and two RX wires and that if it doesn't work with the first attempt then you swap things round and then it works. There's never a complaint because when it works, you have it the right way no matter what the legend on the label or in the data sheets says. When a label/document author gets it wrong, the world doesn't come to an end thankfully and usually it's never noticed or reported because it isn't that big a deal.

If it works it's right! If it doesn't work, swap it. If it still doesn't work then that's the time to worry.

Looking at the two differential signals that might be labelled plus and minus, there is nothing about them that are positive or negative in nature; one is just the digital inverse of the other but both will have the same common mode voltage so it really is an arbitrary decision as to which is labelled plus and which is labelled minus.

Please also read this document that explains that these sorts of errors do occur: -


  What if I'm Unsure?
  
  Reversing the two lines in the differential signal is by far the most
  common initial error when connecting two pieces of RS-422 or RS-485
  equipment. The good news is that it won't cause any damage to your
  equipment to try it the wrong way. If you try it connected one way and
  are seeing garbled data, reverse the leads and try again.