Now to the point of all this. I installed the mounting brackets on the wall 91" from the floor. I checked and double-checked the measurements. I even measured from the wall railing to make sure. After I attached the speakers and adjusted them evenly, I (and wife) noticed the left speaker was lower than the right one. The left one has the high wall and the right one has the low wall. I got out the measuring tape and checked it again. To the bottom of the mount was exactly 91". I tried adjusting the speaker on the bracket and still couldn't get it to look right. I swear it it was a good 2" lower than the right speaker. I ended up having to remount the left speaker higher and later filling in the extra holes.
How can this be? My floors aren't that unlevel. I am on a concrete slab.