The technical answer is "The surfaces in the cabinets cavity at each side and above and below it, create a set of near-field reflections that don't exist when the speaker operates normally unencumbered by cabinetry. These cancel and reinforce at various frequencies, affecting the linearity of the system and producing all kinds of nasty anomalies and unmusical colorations"

A computer once beat me at chess, but it was no match for me at kick boxing.