Sure, here's my 2 pennies.....You have a small bulge on the right that press the surrounding layer of the spinal cord and, since they are saying "presumably" it means they don't technically see this part, but there are 2 small side branches in each vertebra, one to the left one to the right, the one on the right is "presumably" being pinched. Since it is at S1 it would affect your right low back, buttock, and down the middle part of your right leg in the back (not down the front of your leg).
Just so you have an idea of the size of things, let's say your spinal cord is the width of a pencil, these smaller "traversing" nerve roots to the left and right would be the width of the pencil lead.
Hope this helps you to understand better.
cervical fusion 2006, with great result
L4-5 surgery with cages, plates, and screws in 2005, I have continued pain
MEDS: Fentanyl patch, Norco, Celexa, and trazodone
Im going to smile like nothing is wrong, act like everything is perfect, and pretend its not hurting me. (quote used with permission from "Tennis").