For what it's worth, I found this on the web. If it's true, it explains a few things, including why you should use gloves when handling quartz/halogen bulbs.
Quote:
"Over time the lamp filament evaporates and coats the inside of the bulb. This is the origin of the black shadows you see in old lightbulbs. The brighter the light(hotter the filament) the quicker this happens. To lessen this blackening, a halogen such as iodine is introduced into the bulb. The halogen induces the evaporated filament to return the the filament rather than travel to the bulb but doesn't stop it completely. The process is slowed enough that it is possible to greatly increase the current and heat in the filament until glass will no longer work and quartz instead is used for the bulb material, hence quartz-iodine lamps. The downside is that quartz is eroded by the salts and oils on your fingers so the bulb must be cleaned with alcohol and handled with gloves when installing or it can explode."
In my experience, in replacing an old bulb, I tend to do it in pairs, even when using the same exact bulb. If you don't, the old one you didn't replace seems dimmer and more "orange" than the new one. Could be those dark spots/ reduced filament size described above.