In a grow room environment the lamp would be operating in ideal ambient temperatures ~78-F (25-C) that let the surface of the glass get up to its full output operating temperature of 200-F (93-C) degrees.  Lamp output may be diminished if the garden has fans blowing directly over the surface of the glass.  Too much air movement will cool the glass surface and ideal glass surface temperatures won't be reached.  This can cause some lamps to appear dimmer than others.  Redirect the fans and the lamp output should go up.