We report the results of the Monte Carlo simulation study of the fluid phase transitions in micropores. The Gibbs, Canonical, and Grand Canonical ensembles were employed to obtain the adsorption isotherms of the Lennard-Jones nitrogen in carbon slit micropores at 77.4 K. It is shown for the first time that depending on the choice of the parameters of the Monte Carlo simulations, such as the number of particles in the simulation cells, the sampling from the Gibbs ensemble may supply either van der Waals-type or hysteresis loops in the phase transition zones. The existence of these loops is confirmed by independent sampling from the Canonical ensemble using Widom method of test particles. The Grand Canonical ensemble simulations always produce hysteresis loops within the phase transition zone. Outside the phase transition zones, sampling from all three ensembles yield essentially identical isotherms.