Some airports have a non-standard traffic pattern altitude. For example KGXY is 800 feet. Why is this?
Airports underneath controlled airspace may need to keep their traffic clear of controlled airspace, which may start from a typical altitude such as 1000Ft.
As is also mentioned in this answer, in April 2013 the standard pattern height was changed from 800 ft to 1000ft, possibly accounting for a number of airports still having the old "standard".