CO2 levels are off

I took 6 MX1102 loggers outside to calibrate. After a while all were showing values around 390 ppm +/-20, seemed bit low, but maybe these were correct values. Took loggers indoor and set up loggers to start logging in specific dates for 2 weeks, set autocalibration off. Put loggers into the box, drove to the building and made installation. 2 weeks later i looked on data and CO2 values are way too low, like some values were 290. They were in office building, also temp and RH seemed OK. All 6 loggers seems to show too low CO2 value. Loggers are about 1,5 years old. What is causing that mis-measurement?
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