> Scores of authors use the phrase “steep learning curve” or “sharp learning curve” in reference to a skill that is difficult to master. . . . Nevertheless, from the standpoint of learning theory, these and other authors have it backward, because a steep learning curve, i.e., a curve with a large positive slope, is associated with a skill that is acquired easily and rapidly (Hopper et al., 2007).

Source: Fifty psychological and psychiatric terms to avoid: a list of inaccurate, misleading, misused, ambiguous, and logically confused words and phrases (https://www.frontiersin.org/journals/psychology/articles/10....)

Based on common usage of the term learning curve, I had thought of it much like a power curve where the y-axis is the amount of cumulative effort you have to put in to reach a particular point on the x-axis, which measures mastery. Sounds like the official definition is effort on the x-axis and the total amount you've learned on the y-axis, which would indeed invert the meaning from how I've understood it.