This is going to be tricky to explain - I hope this makes sense.
I would expect that if I set the Decimal Places for Unit Types on points, for example, to 0, that my stroke width options would be limited to whole numbers when I use the slider to select a stroke width. What actually happens is that the slider is just displaying the nearest whole number to the actual stroke width.
For example, I set my decimal limit to 0 in preferences, I drag the slider for stroke width to 1, and draw a rectangle. I would assume that the stroke of the rectangle is exactly 1 pt. This is NOT the case.
If I switch the decimal limit to 6, I see that the actual stroke of my rectangle is something like 1.002345. This can also be clearly seen using the pixel grid. The stroke I thought was exactly 1 pixel is visibly wider than grid lines set to one pixel.
I discovered this while trying to make a checkerboard pattern. I couldn't figure out why my rectangles, snapped to the grid, with stroke set to 0, seemed to have gaps. When I changed the decimal places to 6, I discovered the rectangles actually had strokes of 0.000238.
So unless you type a whole number instead of using the slider, you can never be certain that your stroke is exactly what you want it to be. Even with 6 decimal places, sliding the slider to 1 could still actually be something like 1.00000039865. This is very problematic for pixel-perfect exports, and negates the entire purpose of the slider bar.