MCMC Averaged Model Not Fitting Observed Data

I follow the MCMC example ( Multilevel Gravity Survey with MLDA) on PyMC website to implement my own MCMC to solve a geophysical inversion problem. However, I met the problem that the accepted models fit relatively well with the observed data, but the averaged model generated from them does not exhibit the same degree of fit. What factors may cause this problem and how to solve it?