Generally, the big picture is fairly accurate. There are a LOT more candidates for med school than residency, and the further up the ladder, the stronger the application's correlation with good performance and success. Imagine looking at MCAT scores, college grades, and extracurriculars to decide if someone will be a good doctor one day. "Lab assistant" could mean anything from washing pyrex beakers (me) or being first author on a publication (not me). Residency applications, on the other hand, are a slightly better representation of one's ability, focusing on medical board scores, clinical rotation grades and recommendation letters from other physicians.
First, you have to understand the idea of cutoffs....