If it is a new probe you may want to wait a few days and recalibrate. The new probes are stored in a storage solution that is near a pH of 4 so once they are out f that solution and in tank water they will tend to drift for a few days. I am not familiar with the Apex calibration but most have 2 or 3 calibration points. The first one you need to do is the offset calibration. the other one or two are gain calibration points. Since order makes a difference use the recommended procedure. I usually let t sit in each calibration fluid for about 5 minutes or until it quits drifting. After calibration ai go back and check the first point and if it is off I go through the calibration again. Sometimes it takes more than once to get it in since any change of 1 point will have an affect on the others and they may need to be done again depending how much change is made. Hope this helps.
