How is the voltage conversion done? By just the transformer? If i see the input voltage fluctuate the output voltage changes directly with that as well. So my assumption is that its a simple 'analog' voltage conversion done by the transformer.
"Automatic Voltage Regulator" function is typically done using specific design multi tap transformer.
Normally mains current to load just "goes through" its high/mains voltage side, while low voltage side provides power for keeping battery charged etc.
If mains voltage drops significant amount then load is switched to "higher" tap of mains voltage winding boosting voltage.
Again in case of mains voltage rising it can connect load to lower tap. ("buck" function)
Depending on price level there could be multiple taps for multiple levels of voltage boost/buck without need to switch onto battery power.
Again during blackout or when mains voltage goes too much haywire, UPS disconnects completely from mains and runs that transformer the other way using low voltage side to power high voltage side and load.
You'll recognize those UPSes from
scarcity of power semiconductors and actual
old fashioned transformer.
Higher end UPSes again can lack such "dumb" transformer and use more complex circuitry with whole rows of power semiconductors. With what might look like transformer being just big inductor to filter high frequency crud/smooth output.
I'm talking about the Input voltage vs Output voltage, this could differ from 232V input to 235V output. It's compensating with at least 3 volts or so. It flutucates greatly as well (the output voltage together with the input voltage). Worst condition i saw was 231V > 235V, which is 4 volts. The amount of Hz also varies from 48 in worst condition to 50Hz at best. Could the UPS be compensating that as well?
Those few volt fluctuations are down to local power grid voltage varying depending on load and even voltage losses caused in wiring of your house.
Also frequency can well vary some depending on quality of local power grid.
Wouldn't expect very accurate measuring of input/output from cheap UPS.