Why does an encrypted message sometimes end in "=="

As a programming exercise I need to decrypt a message. The only clues that I have is that it seems are: * The encoded message contains only Base64 Characters * a n letter sequence (n in 1,4,7,10) times it returns an encrypted message with "==". * a n letter sequence (n in 2,5,8,11) times it returns an encrypted message with "=". * a n letter sequence (n in 3,6,9,12) returns an encrypted message without a specific character. I do not want a solution, I am just wondering if this sequence of occurrences for the equal sign provide a clue or not.
http://dlvr.it/S233QZ