If the user's AD account profile has never been synchronized to sharepoint, then, when you view a user's profile, it will pull info from AD and show you a simple in-page profile report. If, however, the user's profile has been scynchronized properly, you will be taken to mysites instead.
Note that the AD profiles won't synchronize to sharepoint if you point your USP service to an AD group. In order for synchronization to work, you have to point your USP service to an OU with users, not a security group. Sharepoint will not synchronize user profiles based on users added to a security group. It only synchronizes with user accounts directly inside of an OU.
Thus, to standardize things, either make sure the USP service works and all your user accounts in AD are synchronizable, OR disable mysites completely.