They'll often put a clearance price up and actually include a message of how many units are available at that price. As soon as those sell, they are back to the normal price.
I hypothesize that it has to do with inventory management as discussed above, or just storage. If they have a shelf location in the warehouse that will hold N units, and they end up with N+3 units in stock, they blow out three units and can then have that second shelf location available for some other product. Or maybe they have a target for number of "turns" (units sold per period of time), they'll bump a few out the door to keep on track.
Or something.
In any case, 20 percent off isn't a "get rid of them all" price by any means.
Last edited by OlongJohnson; 02/16/16.