I think I found the problem.
If a pool is 15 x 30 3' -6' its average should be 4.5, on poolmath comes to 15,100.
My pool is 15 x 30 3 1/2 -6', I put the average at 4.75 on pool math comes to 16,000.
But if my shallow end is 3 1/2, it should be less than 16,000, otherwise known as 4.25, or...