was it like that to begin with?
but Rick (above poster) is right, it might bother you aesthetically, but the top of the nut isn't as important to the sound as the level of the strings coming off of it. so even if the top of the nut was perfectly level, if the nut slots are not right, it would still be a problem.
Now, if your nut used to be perfectly level and the slot depths were proper, and then somehow (i cant imagine how) became not level, then I can see how that could cause a buzz... I just cant see how that could happen...
is the wood where the nut is attached to, compressed? maybe the nut got impacted and pressed into the wood below it? again, never seen such a thing, so I don't know if that's reasonable.
if there is no other damage and you find that the buzzing string is too low, you can always build up the nut slot in question with the superglue method (search for a how-to). you may eventually want to replace the nut, but the superglue technique can get you a semi-permanent fix for you.
It's not the size that is important, it's how you use it.