your browser does what is called caching. this means it stores a local copy of things like pictures you see on the internet so if you go to another page a few minutes later with the same picture on it then the browser can use the copy stored on your computer rather than downloading it from the internet again. This speeds up your overall internet usage.
this can have the side effect that when some pictures are changed (such as your avatar) it doesn't always update what you see because your browser shows the old, cached, version of the picture.
Now you have to realise that the picture you see in your profile is a larger version of your avatar than you see in normal threads, therefore it is actually a different picture (despite it showing the same thing). so what is basically happening is you have your old avatar stored in the cache for one size/version of it and the new one for the other size/version.
Most browsers have the option of holding down CTRL and pressing F5 to clear the cache, and doing this will make sure you are seeing the latest version of your avatar on any page you are viewing.