I’ve always thought that the USCS/Imperial dimension of A4 paper was 8 1/4" x 11 2/3" (approximation for 11.69", the exact conversion from 297mm), not 11 3/4".
But Layout (2021.1.278 on Mac) in Document Setup shows the A4 landscape paper size as exactly 297 x 210mm, whether I set the units as Decimal Millimetres, or Fractional Inches.
But if I choose Decimal Inches, the size shows (bizarrely in my view) the paper size in fractional inches - 11 11/16" x 8 17/64". (The fractional dimensions displayed depend on the precision setting - with 1/32 chosen instead of 1/64, the paper displays as 11 11/16 x 8 9/32".)
On further investigation I see that the units displayed for paper size only change if I close Document Setup and reopen it, before checking the paper size in the ‘new’ units, and not always even then. It can take several attempts at changing units then choosing Paper in the dialogue. Merely changing the units then immediately checking the paper size shows the paper size in the units and precision settings that were set before the last time I closed the Document Setup window, and it still doesn’t always update the displayed units for Paper size or show the dimensions in the current setting of Units in the same dialogue.
At the moment, though I have Units set as Decimal Inches, and have opened and closed the Document Setup dialogue a couple of times, and switched between Units and Paper twice too, the A4 landscape paper size is still showing 297 x 210mm (exactly - no decimal point, no fractional mm).
Inconsistent at best, and annoying at worst, not to be able to see the paper size in the units currently chosen.
But I can’t reproduce your additional 0.1 or 0.2mm mm in the paper size shown.
PS. I don’t even get a choice of preset paper sizes in my Document Setup, just the option to type in paper size values to override the defaults displayed. This is with A3 Landscape previously chosen in Page Setup.
And it seems now to be stuck at using decimal inches for the sizes, even though I have set Decimal Millimetres in the Units setting, and closed and reopened the Document Setup dialogue twice.
And when I go back to the Page Setup dialogue, and choose A4 paper size there, then reopen the Document Setup dialogue, this time I DO get fractional mm. And I haven’t changed the Units settings.
Not behaving at all predictably.