Answer:
He does that because he is short and his hand can't reach the 20th floor button
Explanation:
Mitch does that simply because he is short.
When he is leaving, he doesn't need to reach the 20th floor button because the button is way taller than the reach of his hands whereas when he is returning he has to reach the 20th floor button but his hand can only reach the 15th floor button. Thus, he gets to the 15th floor and walks up the remaining 5 flights.