The mean of the the Bayesian IRFs is the mean of the distribution of the IRFs generated when parameters are drawn from the posterior distribution.
When you run stoch_simul after MCMC, the parameters of the model are set to their mean value in the posterior distribution.
But an IRF is a nonlinear function of the parameters, therefore the mean of the IRF is not identical to the IRF for the mean value of the parameters
Best
Michel