People with CO2 Controllers & Experience

I just got my CO2 monitor. It is pictured.

I have a question>
I plugged this thing in, indoors. It reads 1400PPM of CO2 without me adding anything or doing anything.

I moved it outside to check the outside CO2, it reads about 650PPM of CO2.

I called the people that made it they told me recalibrate it outside.
So I did, I turned it to 430PPM CO2 outside.

I brought it backinside, and it now shows 1000PPM of CO2 everywhere in my place, inside. The entire house reads this high.

BUt now the unit reads 350-400PPM CO2 outside since calibration.

Is the unit functioning now? Do I really have 1000ppm of co2 indoors with nothing inside at all? there is no animal inside except me, no stoves or gas appliances. Is this possible? I Dont even need CO2 injection? Maybe its because the plants are sleeping right now? I havnt checked CO2 with the lights on yet. It is 85-90* Outside and I am at sea level.

Its stabilized over the past couple of days to about 1000PPM everywhere inside, and 400~ outside.

Does that make any sense?