"By convention, the first transaction in a block is a special transaction that starts a new coin owned by the creator of the block. This adds an incentive for nodes to support the network, and provides a way to initially distribute coins into circulation, since there is no central authority to issue them."
From whats said here https://news.ycombinator.com/item?id=5548503 miners grant themselves a fraction of a bitcoin for each block, and that fraction is what decreases over time.
"By convention, the first transaction in a block is a special transaction that starts a new coin owned by the creator of the block. This adds an incentive for nodes to support the network, and provides a way to initially distribute coins into circulation, since there is no central authority to issue them."
http://bitcoin.org/bitcoin.pdf