You have a spillover spa that is rasied above the water level in your pool and when the pump is shut off the water in the two bodies of water will try to reach the same level. Depening on exactly how your system it plumbed it is entirely possible that enough water will run back through the plumbing to cause the spa to drop one inch (There is a lot less water in you spa then in you pool so water just running back through the jets into the pipes can cause a noticable lowering of the water level). Try leaving the system off for a couple of days and see if it drops even more. If it does then you might have a leak. In spillover spas the first place I would check are the light niche and the jets.