How to fix cumulative letterboxing/black bars

[CarnageKing1337]

I've noticed that every time I edit 4:3 aspect ratio or even 16:9 aspect ratio videos theres this weird cumulative effect of it adding black bars. I have an example of a 720 x 480P video where I needed to edit something and then I exported it but the top black bar shown on my computer and other display monitors like my external monitor or TV, which is not 4:3, the black bar is bigger. I also put in the right aspect ratio into the editor which is Windows Movie Maker. Also tested it out on Cyberlink Power Director as well which is the same. I tested it out doing it a bunch of times to eventually see what would happen and it literally makes the frame size of the video smaller every time. I obviously just did it once, but how can I make it so that the added black bars aren't there? I have provided sample images from making screenshots from a Surface laptop studio which is a 3:2 aspect ratio. Regardless you can tell the difference on any display.

I circled the areas that shows the effect. The first one is the one thats fine and the second is one I would like fixed.


[Attachment 70935 - Click to enlarge]


[Attachment 70936 - Click to enlarge]

[ProWo]

Cut a 30 seconds part from your source video (not the encoded/exported one) with avidemux and post it here as attachment.

[CarnageKing1337]

So I tried the avidemux and it seems to be fine but now I need to figure out how to get an edited part in there. I need to put some audio into a certain point in the video