Hi again,
I would like to first mention that this metric does not affect you as a seller at all. It's not calculated with other metrics. There are two reasons for this increase = either the different time zones are involved, and even if you replied within 1 hours, across the world the user got it in 2 hours(per their time).
We suggest cache/cookies clearance from time to time, as the number you see is a visual glitch only, and not the bug.
f you have morequestions please let me know while I am here