If you are using a bulb that is made to take a higher voltage than you are exposing it to, it will be less light and will last longer. Because the bulb will not be stressed to it's maximum potential it will live longer. The light output could be significantly less. Unfortunately, there aren't tests done for a particular bulb at voltages different from what they are specifically made for so there is no way to give you a solid answer of how much longer they will live or how much dimmer they will be. You'll just have to try it and see.
Then if you are using a bulb that is made for a lower voltage than what what you are using it for, the bulb will be brighter and will live a much shorter life than it's testing. Simply reverse the reasons above and that's your answer why.
So, you can use a bulb that is not made for the particular voltage that you have just know that there will be consequences.