It’s probably something that might (might!) happen at some point, so since they’re currently retooling the thralls and pets, they put in the level indicator as a proviso. Then they forgot to hide it from the UI. Totally guessing of course, I don’t believe anyone has ever mentioned pets or thralls leveling up in an official capacity. Though there was that comment (don’t remember where, from who or when, so… yeah) about the feeding system being step one in everyone having fewer thralls, which would in turn allow Thralls to be “better” overall.
Could also just be an unfinished move towards indicating the “true level” of thralls, since the 1-4 is not really indicative of their actual power level.
Heh, except all followers whether human or animal have the IQ of a particularly stupid rock.