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With vacuum and gravity bleeding the movement of fluid is so slow, bubbles can hang in high spots without being carried down stream.
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I used to work at a Vette shop. 67-82 Vette's were notorious for causing troubles for folks trying to bleed brakes. One of the cars was giving me fits. I believed I'd received some faulty parts but one of the experienced techs kept saying air was trapped in the lines. He used the example of a fuel filter on an engine to try and convince me that it could happen. Finally I mocked up the fluid system with clear tubing and Tees. As long as the ends of the line remained closed we couldn't make any air appear or remain in the tubes. Brake fluid is more viscous than fuel and the brake tubing is so much smaller that the fluid just seems to push the air out. So I decided I was never going to get hung up on "air is trapped in the lines" after that. Maybe it happens but I tend to look somewhere else if I can't get a pedal and no more air is coming out of the bleeders.
But I have run into cases where air does seem to stick in calipers during a gravity bleed. Sometimes after gravity bleeding the pedal doesn't seem right, and cracking the bleeder while a helper presses the pedal will almost instantly push a few bubbles out.