That seems to make the mistake of summing up the lower bounds to create a lower bound for the total time (and correspondingly sum the upper bounds to create an

That seems to make the mistake of summing up the lower bounds to create a lower bound for the total time (and correspondingly sum the upper bounds to create an upper bound for the total time.) That yields a very unrealistic (as in not-relevant-for-reality) range for the total time. Am I misunderstanding something?