There is multiples way to apply the rounding. When doing the layout, the padding, margin and border are rounded first so that both sides look the same, as they are generally narrow and slight difference would be more visible than the width. Then all the sizes are resolved (for percentages) and added keeping their float value. Finally, they are rounded to the nearest point "on the pixel grid", which is mostly equivalent to Mathf.Round if the scaling of the display is 1.

This keeps the total value of with to be closer to the expected value, as the error could integrate if we were to do it differently. 3 element of 33.33 pixel wide will fill a 100px element without any gap. If all element were rounded to 33, there would be a 1 pixel gap.

This is the same behavior as the browsers, but they do differ in respect to which element is 33 and which is 34 pixel wide. There are no guidelines in the CSS specification for this when we last checked.

To get the exact same size, the user needs to keep whole values and a scaling factor of 1, or have a variable scaling factor but use a rounding similar to the RoundToPixelGrid method available at https://github.com/Unity-Technologies/UnityCsReference/blob/master/Modules/IMGUI/GUIUtility.cs

internal static float RoundToPixelGrid(float v)
{
// Using same rounding constant as GUITexture::AlignPointToDevice
const float kNearestRoundingOffset = 0.48f;
return Mathf.Floor((v * GUIUtility.pixelsPerPoint) + kNearestRoundingOffset) / GUIUtility.pixelsPerPoint;
}