According to a letter from the USPTO "the word 'mini' has been held merely descriptive of goods that are produced and sold in miniature form," and thus, not exclusive to the Apple jargon.
"Registration is refused because the applied-for mark merely describes a feature or characteristic of applicant’s goods," the letter said. "[It is not] a unitary mark with a unique, incongruous, or otherwise non-descriptive meaning in relation to the goods and/or services."
The letter states Apple could appeal the ruling, but as Mashable points out, they would have to come up with a fairly creative argument as to why "mini" isn't just a descriptive adjective.
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