Shelly - measure from the top of the wall (not top of rail) with Rail removed to top of water right at teh bulge, then measure the same about 10 feet to either side, likely you do have a 1/4 " more in that place than on bothof the 10' outside measurements - a bulge is created where the earth wasn't quite flat and is pushing the wall up in that spot - the weight of the water makes it noticeable.
Probably not a big deal, but a pic would really help.