ORP controller question

[christyf5]

Ok this might be a silly question but I just can't wrap my head around this.

I have a Sander ozonizer 0-50mg/hr hooked up to an airdryer and a Milwaukee SMS510 ORP monitor/controller. The controller is set so that an alarm (flashing light) goes off when the ORP goes below its set point. I'm not quite sure what the "alarm" is supposed to do ro me because I'm assuming that at that point the ozonizer turns on and gets the ORP back up to where you have the controller set at. I guess the flashing light makes things more exciting

I have my controller set at 400. The monitor usually reads about 450 or so, I clean the probe weekly to make sure its working properly. However, the only time I see the light on on the ozonizer (ie. its turned on) is when I do a water change and the ORP goes down to about 350 or so. So my question is, if I have the controller set at 400 and the ozonizer never kicks in unless the ORP is below 400. How the heck am I getting an ORP reading of 450?