I'll tell you the reason. You're are 95% correct to have got these strange values.

The voltages are largely affected by surrounding lighting level instead of other circuit element.

If you don't believe me, try hook up just a LED to the DVM on the volt range and then shine a torch into the LED. You can get as much as 1.3V from the action.

Frankly, it is hard to believe that the voltage are affected by surrounding lighting level. Light-emitting mechanism of LED is electro-luminescence. LED is not light sensitive semiconductor device.

later, i will do the experiment to verify.

 

What's the frequency of that?

The frequency is changed with time, from 50Hz,...,100Hz,...,1.25kHz,...,3kHz and then return to 50Hz. It is a loop.

 

Great help!
Thanks to eblc1388, DonQ, Ghar, SgtWookie....