Air buoyancy is the main factor. Cooler air migrates to a hot zone regardless of where the hot zone is located and so an EPA stove has air inlets at the top of the stove , near the flue outlet, & still manages to dive down, airwash the glass, so to get to the hot zone which is the coals on the floor of the stove. The buoyancy of the cooler air is more drawn to the heat of the coals than the draft pull of the chimney.

An unideal wood load burns from the bottom up & as the wood burns underneath, the wood above it gets heated,gassifies,& accelerates in combustion to the point where it may not have adequate air for complete combustion. Try avoid this scenario by raking coals to the front upon reloading the stove.

The ideal fire burns from the top down thus avoiding the acceleration factor previously mentioned. The flames exist above the load & the gases have to pass through the flame due to air buoyancy. All gases within the stove will have higher density than the flame so they have to go to the flame.

When the secondary burn tubes establish a flame atop the wood load, the gasses will have to migrate to this hot zone & the load burns from the top down. With no hot coals under the load, the flaming secondaries become the hot zone on top of the load. Efficient burn of maximum loads is the result.

NOw, imagine the EPA stove scenario described above & turn the image upside down while ignoring the effects of gravity [for this comparison]. A downdrafting wood gassifier is an efficient burner. Gassifier has wood load surrounded by a water jacket to keep the top of the pile cooled & not burning. Instead, the gasses are forced down through the coals [hot zone] & secondary air is added to assure complete combustion. The flame appears at the floor of the gassifier. This is an EPA stove system turned upside down. In both cases, the gases have to pass through the flame before travelling any further & so the gasses get burned, efficiently.