1080, 720, and 480 refer to the height of the video. But if you have, say, an MKV that is 1920×800 (2.40:1) then that MKV is considered a 1080. The reason the height of 800 puts this in the same category as a video with dimensions of 1920×1080 is that they both fit into the same box. The box for 1080p is 1920×1080. So when resizing you are always fitting videos into the 16:9 boxes of 1920×1080, 1280×720, or 854×480.
If you’re converting from 1920×800 to a 480p video, it would be incorrect to change the dimensions to 1152×480, as that exceeds the 16:9 box of 854×480. The correct dimension is 854×356. If you’re converting to 720p, then it would be incorrect to change to 1728×720. The correct dimension is 1280×534.