wheatstone bridge - Difference between Proteus results and KCL-deduced formula

I'm designing a small circuit using an op-amp for a temperature sensor (PT100). I took the design online (see below).

I tried to find out the formula for finding the value Vout as a function of the input voltage. I've used the KCL (I think it's Kirshoff-current-law) at the various junction points. I got a formula that gives values that are offsetted by 0.2Volts from the output produce by a simulation of the circuit in Proteus (original diagram in LTSpice as seen in the picture above). My question is: is it common for proteus to add an offset during the simulation, or am I necessarily making an error in my computations. Thanks in advance