It is just the way gravatar works, its a big distributed network with caching and it is not instantaneous for the image to propagate to all the servers when you create or change your gravatar it may take a while before the new version of your gravatar is on all the servers, also it can be cached by browsers so if you change your gravatar some users may still see your old one for a while.

At least that is my guess based on observation.

Best,

Joe