LTSpice: Not measuring expected value
Unfortunately, this is a known problem that's been haunting LTspice XVII since a year or so. It gets even worse, just try making R1 to be 1, then plot the same differential voltage. Adding .opt plotwinsize=0 makes no difference.
The solutions are:

impose a small timestep, but it can get very small and the effects seem to be linearly decreasing
add .opt method=gear for LTspice XVII from 2020 and beyond
use the version of LTspice XVII from May 6 2019 or before (as it appears in the Help &gt; About)
use the discontinued LTspice IV.