I just talked through the ArticleQuality script and how to interpret it, with another Wiki Education staffer, and we thought something similar would be good to include in the FAQ.
We were looking at the result for https://en.wikipedia.org/wiki/Rachel_Carson: FA (4.7), and the question was, what does the 4.7 mean? (47% likely to be an FA? 4.7% likely to be a FA?)
The answer I gave is that it's based on the probabilities that ORES gives for the possible quality levels: ps://ores.wmflabs.org/v3/scores/enwiki/?models=wp10&revids=877812769
So it says FA because FA is the most likely result (at 42%), but 4.7 means the weighted average prediction is actually closer to GA, with the scale going from 1 (definitely a stub) to 6 (definitely an FA).