It's possibly down to availibility, and it was indeed a 120x90x17. Although when I ordered it, it was showing as a stock item.

The OE fitted rear Pirelli only lasted about 5000 miles, the Avon twice that hence wanted to fit another one.

Trail Attack 3 was one of the recommended alternatives, as were Continental Escapes, and Bridgtstone BT46.

Think they are all aimed at people that ride roads most of the time but who may venture off road from time to time.