Essentially, as you gain crit, the uptime of enrage will increase, which means that each additional chance of enrage is slightly more likely to overwrite a previous enrage effect, instead of completely gaining the whole 6 seconds. This drops the value of crit, and will continue to do so, although very slightly. The value of crit for RBs will stay constant, so it's only the enrage uptime that will further determine the value of crit beyond the static value of procing RBs and crit hits in and of themselves. Hope that made sense.
Basically, as enrage uptime increases, the value of crit will drop in relation.