All furnace manufacturers claim they have the ultimate heating system achieving the most even heating of the glass.

So the glass exits the furnace always perfect heated.

But why looks glass in morning so:

But later the day with identical settings like this:

It is the air temperature and the humidity of the air used to temper the glass in the quench. Warmer air does not transport as much heat from the glass as cooler air, and humid air removes more heat from the glass than dry air.