In any period, such as a quarter, some vendors will discount for a sale, some will achieve the list price and some will exceed it.
When you add all three together you will find a different figure to the average discount because it includes met prices and exceeded prices.
An overall market discount is usually a more modest figure than the average discount e.g. “the average discount in the March Quarter was 6.3 per cent, however the overall market discount was just three per cent”.