I always found it to be from the pipe in the trench settling below the pitless adapter and probably why you got two different depths already. It usually comes from a trencher being use to do trench line that was cut deeper than the pitless adapter. Use 1 1/4 Pex coming out of the pitless adapter to the deeper line (1" PVC sch 40) in the trench(this is the fix for pipes settling in the trench). It's not a frost free fix when the pitless adapter is not below the frost line.