--- That's not it at all. What they're doing when they first turn on is measuring the ambient light, then only allowing enough current to flow through the filament to generate the illumination required to overcome the ambient lighting enough to achieve, locally, the lumens printed on the package. They have to do it slowly, though, otherwise the lamp will oscillate, which is very annoying. Try this experiment for yourself: First, turn on one of the lamps outside on a sunny day and notice how bright it gets when it warms up, then take the same lamp and turn it on in a dark room. Notice how much brighter it gets?
-- John Fields