When you have a mixture of water and alcohol present, the liquid boils at 78.2°C (172.76°F) (water boiling point 100°C (212°F). Pure ethanol boiling point is 78.5°C (172.76°F)).
Air Still/ Air Still Pro pot still mode and Alembic Pot Still:
As more and more of the alcohol is extracted, the boiling point of the liquid remaining in the boiler increases until at the end the liquid reaches a boiling point close to water.
This is why the ABV also decreases over time.
T500/ Air Still Pro reflux mode
Unlike the above, the ABV does not decrease over time due to the re-fluxing action inside the column of the T500. Having the column packed with saddles is what provides re-fluxing. Essentially, the heavier water molecules in the vapour condense and fall back into the boiler only letting the more volatile alcohol make it to the condenser to form a distillate. As the amount of alcohol in the boiler decreases, the flow rate of distillate coming out of the still decreases but theoretically, the ABV of the distillate coming out will be consistent (might drop by a few % at the end but should stay fairly consistent).