This is most likely due to rounding. To get the actual offset, they multiply the scale by the screen size and add to that the offset values. If it’s not an integer, it needs to be rounded, because it needs to be displayed on the pixel grid. I wouldn’t say it’s a bug.
Example: say I have a screen width of 1234 pixels and I want 1%, 5% and 9% of that. That would be 12.34, 61.7, 111.06 pixels when you multiply the scales with the actual width. When rounded, that becomes 12, 62, 111, which are not equally far apart, even though there was 4% between them all in their definition. You can’t really get around this.