I'm fairly early into the game, but while most mobs I encounter have a fixed level display, it seems like a few mobs change levels after several seconds in combat.
For example, the boss in the Amusement Park began the fight at level 16, only to drop down to 15 after I had been shooting her for a bit (I was around level 10 at the time). The air battle with the Goliah in the ruined city also started at level 20, only to drop to level 12 (nearly matching my level) shortly after the first few attacks.
Interestingly, I also came across a pack of Small Stubbys that started at level 12 and "upgraded" to level 14 when they engaged me (level 16-17 at the time?), though I have not yet encountered any other non-boss doing the same.
Since all of these adjustments happen in the first few seconds of combat, it seems like it might be an auto-difficulty adjustment. On the other hand, the game has no qualms about throwing quest mobs or overworld mobs at me a dozen levels higher than my level, so I'm not so sure.
Why are these enemies changing levels? Is it a specific trait for some machines?
I'm playing on Normal, if that makes a difference.