The truth is that she has bangs on every cover besides BTW, and you remember how she said that her bangs are sign of insecurity so since BTW is about bravery that makes a lot of sense.
Well I think on The Fame, The Fame Monster & ARTPOP covers, her eyebrows are covered by her bangs. Born This Way they were purposely "removed" with the use of makeup & prosthetics. Promoting the message of being Born This Way. That's just what I think though...