See here:
This is Cars. It has an aspect ratio of 2.39:1
It will display on a 16:9 screen with black bars on top and bottom.
This is a 1.78:1 Blu-ray - fits the screen fully without black bars.
This is a 1.85:1 Blu-ray - it has small black bars top and bottom.
This is a 2.35:1 Blu-ray - it has bigger black bars top and bottom.
It is all in how the movie was filmed.
Here is a thread explaining it:
https://forum.blu-ray.com/showthread.php?t=16529