A very new user of Draw. My example is - draw a rectangle with Width:3,144mm and Height:167mm.
After the box has been drawn, if I check the dimensions, the Height is now 167.10mm and I cannot get it to ‘stick’ at 167mm. Same thing sometimes happens on other rectangles. My drawing scale is 1:30 and using A4 paper.
With the rectangle selected, click Format > Position and Size (F4). Make sure the padlock is unlocked (unticked). Enter the exact dimensions for Width and for Height. OK.
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Note that if you select a different line thickness than default, then that line thickness (not double line thickness because the line extends inwards by half the line thickness) will be included in each of the dimensions. A 3 pt thick line will make the rectangle 167.10 mm high.
There is not an exact correlation because the underlying measurement unit is twip, that is, 1/1440 inch
I find it easiest to leave lines at default thickness until I have finished any merges or subtractions of shapes.
This is correct for Writer and Calc; but in Draw / Impress, the unit is mm/100. (This is another unfortunate inconsistency…)
But the scale is applied atop, so of course, will affect the possible sizes / coordinates.
You can run up against the limits on resolution in Draw, maybe this time it is converting thirds of mm into binary and back. For higher resolution you can use Inkscape, or maybe Blender.
I use Draw for a lot of drawings because the interface is straightforward and you can generally get things done quickly.
Sometimes I use Inkscape for increased resolution, ungrouping svg images, or more complex procedures.
You may continue your experiments, and check which is the smallest increment that you can have. And with scale of 1:1, that will be 1/100th of a millimeter; with scale of 1:30, it will be 30/100 of a millimeter. I.e., for your example of 2000.10, the previous possible size would be 1999.80, and the next one would be 2000.40.
Is there a help concerning ...>>Scale>>Drawing scale ?
Clicking the Help button, I find none.
(I also never had a plausible reason to look for such an option or to use it. And the term “scale” doesn’t seem to tell what actually is achievable.)
tdf#165141
This wasn’t too sleepy. 
I feel it does… it provides a scale, which maps the objects’ sizes/positions to the chosen paper size.
That’s probably somewhere above my head.
I have done some experimenting and wondered whether there was an issue with a Drawing Scale of 1:30.
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Create a new Draw document and set the ‘Drawing Scale’ to 1:20. Draw a rectangle and change the Width to 2000 mm and Height to 2000 mm. Select OK. Now go and check the width and height. They are for me W:2000 and H:2000 (which is what I want) Close LibreOffice.
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Create a new Draw document and set the ‘Drawing Scale’ to 1:30. Draw a rectangle and change th Width to 2000 mm and Height to 2000 mm. Select OK. Now go and check the width and height. They are (for me) W:2000.10 and H:2000.10. I cannot change the Width and Height back to 2000 mm and for it to ‘stick’.
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