There is an issue in Adobe Illustrator. If an artboard position isn't a whole number (has digits after comma in x or y position), Illustrator automaticly adds 1px to width and heigth when you export for screens in 1x resolution. That's wierd.
It may well be weird, but it's what we have to work with at present.
Illustrator appears to export its pixel grid from coordinates of the whole pasteboard, rather than treating artboards discretely, even with Export for Screens.