An additional thought, is that perhaps the Classic has not been configured for the SOC to reset to 100% (or less, if the battery is below the Reference Temperature), when the Classic transitions to Float (on its own).
This, plus an efficiency that is not reasonably correct, could cause the shown SOC to decrease each charge day.
The following was stolen from the Setup section of the WBjr Manual,
" ... The Net Amp-hours tally can be configured to reset every time the Classic goes to float. To set this option, go to the “TWEAKS” menu and press “MORE” four times. Select “WBRST” and press the UP or DOWN arrow keys to select “YES.” Press Enter to save. “NO” (or no reset) is the default ...".