It sounds like you have an Xbox Live profile and also an offline Console profile. You can attempt a gamertag change on your Xbox live account either through the console or the website and see if you can get the name you have on your xbox, but chances can
be slim, as if the tag is already taken, you won't be able to use it.
But the reason that this is happening is because you have access to two different types of accounts, and each has different rules when naming the account.
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