I did have a theory and your measurements bear this out (a bit). Now this seems far fetched and I do not really see how it should happen, but. I noticed with WWB that the feed ramp and chamber got really dirty. In theory this should not happen as burnt gases should escape, for the most part forward. What I considered that what if WWB was expanding enough to nearly seal the chamber and hold the case, but was letting enough gas escape to the rear (while case still expanded in chamber). This would cause a 'soft' slide movement, but a 'hard extraction' (case still expanded). This would correctly and safely cause the extractor to release the case prior to extraction.
Of course the other theory is that the slight difference in case size allows gases to escape to the rear prior to enough case expansion, the slide movement starts and then the case expands enough and locks, causing hard extraction and extractor to release the case. Basically the slide movement, case expansion and extraction all happen in the wrong order due to gases escaping to the rear too early.
I only say this because of the circumstances; the difference in case size, dirty feed ramp/chamber, my cases extract about 1/4 inch from chamber before extractor releases them (still in chamber). Theory being that if the extractor manages to pull the case that initial 1/4 inch, it should not release it until ejection unless there is a force acting against it (late case expansion).
Just a thought and theory, although I know it is a bit 'out there'.