Probably a combintation of all of the above (dark adaptation, ratio of old/new bulbs, deliberate adjustment). You gotta figure that Disney doesn't just routinely change out all the bulbs, they probably have a light meter or a person who "decides" when it is time, or a certain ratio of out/dim bulbs at each fixture. So there can be periods where most of the lighting is old/non-functional and then new and fresh which should produce a distinct difference in the level of lighting. I doubt most rides have lighting that can be "dialed up/down" but there may be some areas that can be (other than overhead lights). Plus maybe they are switching from incandescents to LED or some other technology which affects the lighting level.
Maintaining consistency of such a user subjective thing like lighting over an entire complex ride that runs 10+ hours a day, EVERY day, has got to be a nightmare.
Maintaining consistency of such a user subjective thing like lighting over an entire complex ride that runs 10+ hours a day, EVERY day, has got to be a nightmare.