@Modawg2k I dug in a bunch when I was changing out heads. The deepest I measured was ~ 7 inches, but the ones I saw extending that deeply were certainly fewer than those that were higher up. I'd say I had a greater density through about 4-5 inches if I remember right, but definitely had some down to 6 and fewer still to 7.
Maybe what you're seeing is the rhizomes, which if I read these articles properly is different from the roots.
The 1st article is actually about getting rid of Bermuda, but note the description of stolons and rhizomes, then this comment:
If you can feel the difference when you are under the mat of sod. Cutting into sod is tough, Beneath should be easier, surprising easier. You will probably see horse hair type fibers still sticking up. These are the roots."
This article talks about watering 8-10": http://southwestgardener.com/summer-lawn-care/
There's probably more articles out there, and some probably conflict these. I wish it were all more obvious!