To carry on from Genghis' thoughts, this question periodically comes up in Oz-land as well.
I have both a BE (as against Genghis' PhD .. so he can talk more to the university standards stuff) and ATPL and, as well, taught CPL/ATPL theory for quite a few years in my earlier days.
Points I would suggest for consideration
(a) ATPL content is comparatively straightforward and cognitively similar to mid-secondary level standard
(b) ATPL exams (and I'm only talking Oz, here, and then, specifically, looking at what existed in the 70s/80s) difficulty lay in the speed and accuracy department. Pass mark requirements are high and, for most subjects, the work has to be known to a manipulative standard sufficient to permit the student to read, assess, get into it, and get it done .. pretty quickly. There is no time available for figuring things out from first principles .. either you know it .. or you fail due to not completing enough of the exam.
(c) undergrad exams tend to acknowledge a generally uniform standard amongst students from year to year so there is far less emphasis on an arbitrary pass mark per se. Hence the well known "zero shifting theorem" for marking protocols. While one still has to arrive at a suitable answer (otherwise the bridge might fall down) a lot of emphasis is on interpretation and understanding.
Overall, undergrad and ATPL assessment are black and white in their differences .. comparisons are bound to be arbitrary and unsatisfactory.