Why did the round face become a long face?

Kentaro Hoshino witnessed a neighbor's murder through a crack in the window. Because the murderer flashed several times in front of Hoshino who peeped through the window, Hoshino clearly remembered the man's face.

Hoshino described the witness as a man with a slender face, but the murderer who turned himself in in the future was round face, not slender face.

Isn't the person witnessed by Hoshino the murderer?

Answer:

The person witnessed by Hoshino is the murderer.

The murderer is a round-faced man. So who is the witness of Hoshino? To dispel this doubt, let's do an experiment.

If there is a round-faced person around you, you draw a long hole in the middle with a piece of paper, and then let the round-faced person walk around quickly in front of your eyes, so that you can see a slender face through the gap instead of a round face. This is just an illusion.

Hoshino mistook the round-faced murderer for a thin-faced person, because he saw it through the slender gap in the window.