I'm not positively sure on this, but I think that mercury vapor bulbs gradually lose brightness as they age. While giving less light, they still draw the same power.
Look at the reflectors inside older light fixtures. If they are not shiny bright, part of the light is not reflected out. This further reduces the fixture efficiency.
My employer changed out all shop lighting fixtures several years ago. The new high efficiency fluorescent fixtures put out more light and use less electricity. They now need to run the heating system in cold weather, where before they were cooling almost all winter.
And Oil Pan is right, you must match the bulb to the ballast.
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