Why do ratings and percentiles are generally decrease with game clock time?

(Sorry for the typo on the subject)

It seems that generally people have higher ratings and percentiles in variants when the clock time is higher. Do you agree it is a common pattern? How would you explain it?

(I would remark that the intuitive explanation "you are obviously stronger if you are allowed to think more" doesn't make sense: percentiles don't measure your absolute strength, they measure your relative strength compared to others who play the same variant.)

The stronger players play the faster time controls.
Hence it is normal to have higher ratings in the slower time controls.

It would make sense assuming that
* the stronger players do NOT play slow games
* the weaker players do NOT play fast games

but why should it happen?

Generally the stronger players play faster time control only.
One reason is computer assistance at slower time controls.
Another is that the strong players play slow games over the board and play bullet games online for fun.
The weaker players cannot even play fast time controls as they either time out or blunder.