Grove AC Voltage sensor measurement

Hello,

I recently bought a Grove AC Voltage Sensor but I can’t get the real voltage value, when I send 220V, I have 220V but when I send 150V, I measure 180V and when I send 120V, I measure 147V. ​​Can you tell me where the problem is?

I suspect it’s due to the line: meter.VoltageStp(39, 47, 0);

In addition, I would like to get the AC value of the voltage next to it, what program should I send to receive it?

Thank you
Sincerely
G.Drouet