The experience of actually playing for a while helps answer this question.
Poker isn’t usually like what’s shown on TV.
After you’ve been grinding away for 8+ hours, hands run together. You lose focus, the people at your table have changed multiple times, the dealer has changed multiple times, you get hungry/thirsty, you’re tired, your back hurts from bad posture or seats, etc.
It’s incredibly difficult to be at the top of your game for so long. Sometimes you slip up. There have been times I played my previous hand, rather than my current hand after sitting through hundreds of deals.
Most players are good enough to remember what they had after one look, but it’s really easy to let your concentration slip, especially if you’re in it for the long haul, rather than expecting a a few big hands in a short amount of time.
And that’s actually a common physical tell. Fatigue happens to everybody - even the top pros. They aren’t, for the most part, damning, but it gets to be obvious when some players just aren’t in the game anymore. You have to adjust your play in response, i.e. if they look at their cards twice, it may just be fatigue or disinterest rather than a deliberate ploy.