Reward system bitcoin
The amount of Bitcoin a successful miner can claim for himself is currentBlockReward + transaction fees. If there are a lot of fees, a block can be worth more than others.

When users send transactions, they can attach a transaction fee by assigning less to the transactions' recipients than the inputs' total. The unassigned amount can be claimed as transaction fee: Inputs - outputs = Transaction Fee

Example: In: 0.005BTC, Out: 0.004BTC ⇒ Transaction Fee: 0.001BTC


When miners work to confirm a block, they include their individual Coinbase Transaction in the transaction data that they are trying to confirm. This Coinbase Transaction spends the block reward and transaction fees to an address of the miner's choosing. This means that a miner assigns his own reward when he successfully finds a block.

However, after a block is found, every node will check whether the block adheres to the rules: The sum of all transaction outputs in a block must be smaller than all transaction inputs and the block reward, i.e. the miner may only assign the free floating transaction fees and the current block reward to himself.

sum(BlockOutputs) ≤ sum(BlockInputs) + BlockReward


It has happened in the past that a miner claimed less than he could have. In that case the remainder is lost to the network.

Also see the related questions:


Who gets Bitcoin transaction fees?
What exactly is Mining?