Operational amplifier input voltage
Seems like you have the output railed and you are just seeing the highest voltage that poor op-amp can manage with the supply voltage you are giving it.

If you are literally putting the same voltage on both inputs with no feedback it will almost surely either be railed at the positive or negative limits. That's because of the high gain (400,000 typical) and typical +/-250uV offset, which means the opamp should be putting out +/-100V typically.