Why does it seem out of kilter? PSA doesn't usually have any correlation with Gleason score and doesn't always have any correlation with seriousness of the problem.
A man can have a PSA of 80 and no cancer, or a PSA of 3 and extensive bone spread. He can have a PSA of 15 and is just as likely to have a G5+5 as a G3+3.
It is all just part of a picture the medics put together to assess the potential risk for each individual patient. It seems that in your case, they have found only a small amount of low grade cancer so want to monitor your PSA for a while before dragging you into life changing treatment.