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Despite being in the same course, students’ initial performance varies substantially from about 55% correct for those in the lower half to 75% for those in the upper half. In contrast, and much to our surprise, we found students to be astonishingly similar in estimated learning rate, typically increasing by about 0.1 log odds or 2.5% in accuracy per opportunity.It sounds like students were pre-tested, given instruction, and then post-tested. The researchers assume that students who did better on a pre-test were solely leveraging prior knowledge. The researchers do not consider that higher pre-test scores could have been due to the ability of some students to discern the correct answer from context, analogies, or first principles. Students who are able to do so would be seen as "quick learners" by their teachers and peers — though not by these researchers. These students would not need as much time to master all of the content in a lesson, since they would grasp perhaps 50% more of it than their peers right out of the gate.
The researchers focus on the fact that these students appear to learn the additional bits at roughly the same rate as their peers. But of course, if they have much less left to cover in a given lesson, then they will be done with the lesson well before their peers. By most definitions, this makes them 'quicker learners', assuming their initial performance was not based solely on prior knowledge, but was also based on their ability to figure out some of the material from the initial questions themselves.
Also, one would assume that the last parts of a lesson are the hardest to learn, so a student who understood 90% of the lesson at first might still be learning the last 10% faster than a student who came into the lesson understanding 50% of the lesson (when such student gets to the 90% point). However, the student who student who starts at 90% might appear to learn at the same rate as the other student because the other student is getting the 'low-hanging fruit' and moving from 50% to 60%. This should not be considered equivalent to moving from 90% to 100%.