Connectionless mode makes it so the camera is being either distance or time triggered by the aircraft. This means that it will possibly take pictures as the aircraft is slowing down, turning, and speeding back up as opposed to just taking images at the cruising speed within the survey area.
The estimates are just that, estimates, and they shouldn't be too different between Connectionless and Active modes.
If your survey area polygon is concave Connectionless mode will take images inside and outside the polygon while Active will only take it inside. I now see that this is not being handled well currently and we will fix this in the future.
Can you provide screenshots to show the difference you are talking about?