Tag Axle Tires

I was looking at the tires on our new Marquis and it suddenly hit me that the tag axle tires were a different model. All the tires were replaced 3 years ago with Toyo 12R 22.5 model M117z except the Tag tires are Toyo Model M320Z.

I thought this was odd since the M320Z is on on road/off road tire while the others are your typical all position tires.

I'm curious if anyone has an opinion on why a tag would be different.