What if it is because the 250w bulb does not appear as concentrated? The 250 would saturate much faster then a 400w and could look more "washed" out or "spent" faster thena 400w in the same space. IE: it could be an intensity issue. Put a 14K 250w in a room and turn it on. It's only going to be so bright and the color will "fade" more quickly. Put that 400w in the same room and turn it on. IMO, it will be much brighter and the color should be more dense.