Hey man I just got a PPM-3 controller also and I was wondering about similar issues.

When I got it read 1400PPM indoors and 650ppm outside.
I called them and they had me re-calibrate the unit.

Now it reads 400PPM outdoors and 1100ppm inside...

I thought this was a little odd, I mean I thought CO2 levels should be 400-600 indoors just like outdoors, already so high and I havnt added anything?

also when I Switch on my A/C my co2 climbs, somewhere around 1300PPM just by using the a/c. weird?

Kinda feels like I didnt even need to add co2 lol.