Their 100 W bulb only puts out the light of a 75 W, that is, about 1250 lumens. The 75 is equivalent to a 60, the 60 is a 40.
In other words, these are lower efficiency bulbs. You need more bulbs to get the same light or you get used to dimmer lighting.
They don’t get as hot so they put out less light and last a lot longer. Also, the light is ‘warmer’ colored.