Geometry. With the strings capoed, the "triangle" of the level of the fretboard and frets, to the height of the strings, and back down the string to the capoed fret, has a greater angle on the string going up than does the longer distance from the nut to the bridge, up to the strings, and back down to the nut.
It's the same as when you are close to a building. You look up at a greater angle to the top of the building when closer than you do when you are some distance away from the building.
This is one reason that many manufacturers specify some sort of relief to the neck, in order to give the string room to vibrate. Because the longer effective vibrating length of the string its full length will result in greater string excursion than the shorter effective vibrating length of the string when capoed.
These two concepts together make for the difference fret buzz versus no fret buzz on any instrument.