That’s easily explained - in part - by the different default calibration constants.
If you’re running the EU adapter on 220 V, the “USA” setting is wrong - you’re effectively saying that 9 V out (actually, it isn’t, it’s closer to 11 V, but the argument holds true) represents 120 V mains whereas with the UK/EU setting, it represents 240 or 230 V respectively.
What I can’t work out is how that gives you 260 V, it should show something like 110 - 115 V.
The calibration details:
The EU adapter is 230 V in, 11.5 V ±5% out (at 50 Hz). What it will be at 60 Hz, I don’t know. But it won’t be very different. That is divided down inside the emonTx in the ratio 1:13 and fed to the ADC. Those resistors are ±1%, the error in the divider might be about 1.2%. The calibration constant is the mains voltage that would give you 1 V at the ADC input, (not the actual voltage) so for you, it should be 260. You could expect to change it by up to ±6.25% to get the correct reading.
All this is assuming you don’t have a fault in your emonTx - all three? - unlikely.
As a first step, set the DIP switch to “not USA”.
Next, comment the voltage calibration constant “Vcal” in line 99 and uncomment the one for 260 2 lines below, and delete “const” at the start of the line.
Check what the result is - it should be closer, and then if necessary do the calibration procedure.