The reaction of the scrubber is also producing H2O some of which will be water vapor in the loop. So there is additional gas entering the loop with the scrubber reaction. The solid sorb granules themselves contain about 20% water before reaction and this gets released by the reaction.
Soda lime - Wikipedia
The reaction mechanism of carbon dioxide with soda lime can be decomposed in three elementary steps:
1) CO2(g) → CO2(aq) (CO2 dissolves in water – slow and rate-determining)
2) CO2(aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)
3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH ((NaOH recycled to step 2)
If that were the explanation, it would be present at all phases of the dive, not just on deco. I can hang out at a constant depth when not offgassing all day and maintain PO2 simply by adding a little O2. My scrubber is not adding inert gas to the loop in any meaningful quantity.
Edit: Plus, water vapor does not increase indefinitely. It condenses and is flushed out, or you accumulate a little in the counterlung in liquid form. A breathing loop is probably already pretty much 100% humidity. This isn't the explanation for the PO2 drop.
I think it is the solenoid action in an eccr which leads to buoyancy effect and thus noticing the volume increase here.
Not for me. I'm running manual on deco. Plus, the solenoid is only injecting O2 and if inert gas were not being added to the loop through offgassing, loop volume would remain the same, only slightly oscillating as O2 is metabolized then replaced to maintain a constant PO2.