Hi Milton,
I haven't used this myself before, and so I was gonna wax on about my best guesses, before sending you to the new help portal (see below) for faster help...
But then it occurred to me to just check the plain code-file for the tutorial: If you download that (on the top left of the tutorial page, called mcmc_state_dependent_BM.Rev), and look at the section in question there, it absolutely clamps the disc data to X on line 55. So I think that line just got lost in the narrative version of the tutorial.
It is meant to replace this google group and should be more monitored, so you might get faster help there.
Cheers,
Orlando