I do not want to sound rude or dismissive but I agree whole heartedly. Leave it alone. I do not place too much effort on thinking about what these gauges do. I glance, and move on. I take what they tell me in advisement. Focusing too much on these gauges can lead to a wild goose chase, multiple page threads, and lighten your wallet by $500 or more, for nothing. I've had multiple GM trucks across three different generations and they all do this, bounce all over the GD place.
There sure is a possibility of a weak O ring, or clogged pick up strainer, etc, but you'd see more concrete, blatantly wrong readings.
If your mechanic is telling you not to worry about it, which when you think about it hes turning business away from himself, listen to him. It's not worth it.