I think its exactly how you described it. The seller has to be the last to respond at all times. Very silly. Not sure if this is true but i have had someone else tell me that. Along with a never ending conversation i personally had with another seller. She insisted to be the last to respond. It was quite silly but it proves my point that you assumption might be correct.
Earlier today i checked my response rate. It was 94%. The next time i logged on it said 80% (This was within the same day btw). I got a message shortly after which i promptly responded to and it immediately jumped to 90%… Not sure what thats all about.
BUT i dont think its something sellers have to worry about. From what i have been reading so far even top sellers have very low response rate. I saw one guy that had 27%