Thus, what's the effect of the Latin prefix 'ab-'? What are the similarities and differences?
What's this phenomenon called, in which a prefix or suffix affects nothing?
What are some more accurate words that characterise two words, X and Y, for which all of X's defnitions are contained in Y's, than 'subset'?
They're more or less synonyms. Aberrant is more used to describe deviation from moral/societal norms. Errant has a stronger association with "error", so can be used for people prone to make errors. ("Aberrant", "errant", and "error" all come from the same Latin verb "erro", meaning "to wander".)
You have already noted the formation of the words and the significance of the prefix ab-, so where's the issue now?
err: to stray, OFr: errer
ab-: away; from
Errancy refers to a system's nature of deviation. Aberration is the instance of deviation. Errant behavior should be treated by behavioral correction, an aberration needs to be remedied by restoring to normal.
Also, the ab- prefix is generally not used with a positive connotation, while error by itself has no such baggage: "To err on the safe side."