I am sure it is the temperature as Poolsean said. I have an Aqualogic PS-8 and it controls my pool and spa separately. I know the salt concentration in both is the same (I run the spillover when adjusting chemicals and check salt levels in each with both chemical tests and a handheld and calibrated salt meter) but my spa constantly show a higher salt concentration than my pool on the Aqualogic control panel (usually about 200ppm higher). The only difference is my pool is at 82 deg and the spa at 104!