There are three obvious problems with your circuit.
1) Assuming you want a differential amplifier, you need to feed back the output to the - input, not the + input. To see why this makes a difference, add a little AC to V1, and watch the output swing from rail to rail. As part of the simulation process, the simulator has produced a solution for inputs/outputs at turn-on. This does not imply that the solution will remain stable for changes in inputs, and if you change them you will see that your circuit does not have a gain of 10.
2) You are grossly violating the MAX4239 input common mode limits. Your inputs should be no more than Vcc - 1.3 volts. See the data sheet.
3) You are violating the maximum supply voltage rating for the MAX4239. Maximum total supply voltage is 5.5 volts.