You've played a variety of games a different time controls, so no one answer will be satisfactory here. Some possible explanations are as follows:
a) your clock is moving properly but your opponents simply play faster than you,
b) your opponents are using premove more regularly than you are,
c) the correction for lag for either you or your opponent is not working correctly,
d) you are imagining all this, or
e) your opponent is moving at near light speed and time for him slows down but passes quickly for you.
Occam's Razor would suggest that a), b) or d) are better choices than c) or e). But I wouldn't rule any of these (or other possible explanations) out.
For some reason, no matter how fast I move in Live Chess, my time limit is always dramatically lower than my opponents. Any idea why this might be?