The manual may figure the bike was originally jetted from the factory to run Yamaluba at 20:1. I had numerous yz's in the late 70's and early 80s' and the manuals on my bikes all recommended Yamalube at 20:1. Of course I was running smaller motors back the and I never had problems with Yamalube(or similar oil) at 20:1. Back then my Dad thought more premix (more money)was still less than an engine rebuild from lack of lubrication. My older brothers routinely ran very little premix and always had to rebuild their motors.
When I went to bigger bikes, 500cc, I used to run 40:1 and never had problems. Like CharlieT noted, smaller engines run higher rpm's and need the extra lubrication while bigger engines run lower rpm's and need less lubrication.