Because Anki implicitly assumes that all lapses are caused by an inappropriate ease factor, when in reality a large portion of lapses are caused by the normal functioning of the forgetting curve, a certain number of cards will have their ease factor unjustly reduced after a lapse. This means that the lapsed card's interval will grow more slowly than necessary from that point onward, needlessly increasing the frequency of review, and in turn wasting the time of the user (the ease factor problem).
Now, one could argue that a corrective mechanism to mitigate this issue is already built into Anki: the "easy" button. Theoretically, if the ease factor of a card was unjustly reduced due to a fluke lapse in memory, then the next time that card comes up, you should find it easy to answer, and therefore grade the card "easy" and restore the ease factor to its correct value. The logic here is that an unjust low ease factor will cause the algorithm to underestimate the appropriate growth in stability (interval), in turn causing the card to show up before retrievability has dropped to the standard amount. Notice the implicit assumption here that retrievability (probability of successful recall) directly correlates to the subjective difficulty of recalling a memory. But this is simply not the case. Retrievability, which is what Anki's algorithm is actually predicting, may indeed be indirectly correlated to subjective difficulty of recall, but it's definitely not a one to one relationship; subjective difficulty of recall will also be highly influenced by things like the time of day, how tired you are, and your general mood. Not to mention the ever-present possibility that the only reason you found a card easy to answer in a specific moment was due to the same sort of memory fluke that I talked about in the previous section. In fact, ja-dark actually cited studies that found that people's subjective judgments of how well they know something are highly inaccurate. So, just because a card felt easy to answer at the specific moment you reviewed it, that doesn't necessarily mean that it is less intrinsically difficult than Anki had previously thought, and thus can safely have its interval grow at an expedited pace. If you mistakenly grade a card "easy," it's extremely likely that its interval will grow too quickly, leading you to lapse it in the future. So, by attempting to utilize the "easy" button in order to counteract the damage done by Anki's excessive "learning rate," you are risking causing more damage than you are mitigating.Previous: The Ease Factor ProblemNext: Solving the Ease Factor Problem