It has to do with how your auditory apparatus perceives sound. Up to 15ms every sound that flies to your earholes gets perceived as a single sonic event. This effect is pronounced up to roughly 25ms, when, depending on frequency you will start hearing smearing. Stuff that arrives in 30ms and later starts to contribute to the "room" sound and the feeling of envelopment. This is basically the mechanism how reflections alter the tonal balance and imaging.