\cmidrule's trim option

Your interpretation of the trim option is correct. What seems confusion here is the fact that removed the column space on the outer edges of your tabular.

When considering the tabular version without the end \tabcolsep removed

\begin{tabular}{ l l r }

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the adjustments for \cmidrule using the left and right trim options seem more in line with what one would expect.

The default trim, if not specified explicitly is \cmidrulekern which is set to .5em. This equates to 5pt under the 10pt (default) document class option, 5.475pt under 11pt and 5.87494pt under 12pt. Perhaps, instead of specifying trims in absolute values, is a font-related width like em.