Thanks for your reply, James. The other multimeter is an economic one. So it is entirely possible that the cheap multimeter’s readings are below the actual mains voltage.
I tried your suggestion and it works! So immediately after holding my phone sideways, I press the refresh button, and I can see the sine wave. When the sample frequency is 4000 hz, its peak voltage is about 190 V. So the RMS calculations are correct. But can it be possible for the mains AC voltage to go as high as 137 V RMS?
I compared the mooshimeter’s VDC readings with the other multimeter and they are identical, which is great.