I think there's a whole level of complexity being missed here... Notice that tenacity and potency are not just percentages, but +x%, which implies its relative, just for starters ;). But ignore that for now
But to keep this simple, the problem is you dealing with statistics. 80% chance refers to the AVErage number of sucesses seen in a set of trials... An easier way to put this: they're saying if logged 1000 fight a day, and tried dodging 10 times in each battle, each battle has a number of sucesses, each day has a different percent of successes. You take daily percent dodged from 0-100%, each day, and you have to average these PERCENTS. The more days you add, this last average should get closer to 80%.
But the real kicker is statistics law says that you might get all 20% of thassumese fails first, which is all just a fancy way to say that if you aren't seeing 80%, you haven't done enough days, AND numbers law assumes an infinite number of days!!!