It could be a 4/3 SD content broadcast on a 16:9 cable HD channel. it could be a 16:9 program reformatted to fit a 4:3 TV and then played back on a 16:9 TV producing black bars all around. The cable company could be broadcasting a 2.35:1 movie with horizontal or vertical black bars (or they might not). They could have butchered the content by chopped off the top and bottom of the picture or stretching it indiscriminately. The list of possible permutations of all of these crimes goes on and on.
Basically, cable TV is a mess when it comes to formatting and aspect ratios. Every channel has a different approach. The auto format function on some TV can help but it won't solve every issue. If the zoom and formatting command of your remote isn;t handy you could get a universal remote and map the function to a easy to access button.
I have cable service from Rogers but the incoming picture does not fill the whole screen. I need to change the settings to "stretch" every time. I have two questions, why does the picture not fill the whole screen and is there a way to lock the settings to "stretch so that I do not need to change the settings everytime I turn the TV on....thanks