From Wikipedia:
Reverberation is the persistence of sound in a particular space after the original sound is removed. A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air. This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude, until they can no longer be heard... In comparison to a distinct echo that is 50 to 100 ms after the initial sound, reverberation is many thousands of echoes that arrive in very quick succession (.01 – 1 ms between echoes). As time passes, the volume of the many echoes is reduced until the echoes cannot be heard at all.
Okay? What you have to keep in mind is that these millions of millisecond and microsecond "echoes" are bouncing off of every hard surface in the room in every direction.
Now you have to take into consideration how shotgun mics work.
From Wikipedia:
Shotgun microphones are the most highly directional. They have small lobes of sensitivity to the left, right, and rear but are significantly less sensitive to the side and rear than other directional microphones. This results from placing the element at the back end of a tube with slots cut along the side; wave cancellation eliminates much of the off-axis sound.
Even though it says "left and right" there are also reflections from up and down.
Are you with me so far?
So because of it's narrow polar pattern a shotgun mic picks up, primarily, the sounds directly in front of it. Since the shotgun mic is pointed at the sound source the sound reflections that are being picked up are the ones that are bouncing off of the hard surfaces behind the sound source, so they are the ones that have traveled the furthest. So because of the way the shotgun mic works, capturing only the longest reflections from behind the sound source, the shotgun mic actually exaggerates the reflective qualities of the room.
Clear as mud, right?
The last thing you have to consider is that a microphone is picking up a very directional way. Your ears pick things up in a spherical pattern, and then your brain actually "edits" the way that you hear things.
Here endeth the lesson.