In the mentioned plot, ReplayGain is 1LU louder than R128!This is contrary to my results (R128 ~1LU louder than RG).
edit: R128Gain rounds values to one significant digit before it writes them to the file, while foo_r128scan and r128-* use 2 significant digits.
foobar2000 reports five significant digits...
foo_r128scan already logs in full double precision floating point values, but display and possibly tags are limited to two significant decimal places. I don't know where you are seeing these five significant digits of ReplayGain data, but the current version uses the same fixed two digits in the SDK when converting the floating point gain values to text for writing tags.
it's .01 variance from your plugin