I don't know the details, but TCC can be set to look for external things before looking for internal things. I think it's done with aliases, i.e., if you create an alias with the same name as a TCC internal command, TCC will first look at and process the alias and then won't run the internal command.
As an analogy, Rex told me that if you set _date=01/05/2015 when you echo %_date it will return the value of the variable you created instead of the value of the internal variable.