As elcius points out, sizing is off on every UI element due to UI scale. If you don't use the UI scale option, WoW does not default it to 1, it attempts to give you a pixel perfect size, which is about 0.7 for a 1920x1080 screen. In the past, WoW gave you 1, which is why older screenshots, especially vanilla, had bigger screen elements like the bottom bar taking up the whole screen width.
The problem with a UI scale under 1 is that while sizing and positioning attributes go with the screen size, the "absolute" values are hidden and configured for us behind the scenes, which are approximately "screen / scale". This means a 1920x1080 screen using a scale of 0.7 has an absolute size of 2742x1542 "underneath". There is no way to directly get these values, at least no way I'm aware of, but MoveAnything gives you the absolute values of a frame, which is what I use when carefully moving something to avoid weird thick or antialiased borders. The "visible" values end up with decimal places but the frame I'm moving gets a razor sharp edge.