Mobile data use not accurate

I have set up android and a utility meter called mobile_rx_gb reset daily
However I find it’s not right. Eg one day I had data off whole day. The mobile should be 0, but mobile_rx_gb said 0.05.

Other days, I see mobile_rx_gb go up at 2am, again when data is off. Other days it’s just wrong. Eg yesterday my plan said 260mb, the mobile_rx_gb says 0.51

What is the reason for this?

The data reported is exactly what we get from the API so that is what we get