If Chicago courses are, on average 1.2 times as much work (and presumably content) as Northwestern courses (based on the stated equivalencies to quarter hour credits), and the Chicago student took 21 courses due to expected overloading in three out of six quarters (rather than 18 courses by taking all standard load quarters), then the Chicago student will be ahead on credits compared to most students at other universities, but not ahead on credits compared to students at other universities who take similar higher than standard loads (to reach 105 quarter credits in six quarters or 70 semester credits in four semesters).
Presumably, Chicago has decided that “one course” should have an amount of workload and content that is 1.2 times larger (on average) than what Northwestern puts in “one course” (on average), based on each school’s conversion to quarter hour credits.
Note that some colleges have more variable credit courses. For example, Stanford has courses ranging from 1 to 6 units (quarter hour credits). Presumably, they (and other schools that have similar varying amounts of credit per course) do not believe in force-fitting courses into a standard size in terms of workload and content, but sizing courses based on what they believe are useful sizes to slice up a larger amount of content into.