3
It's an empty block (zero transactions), but the coinbase transaction is 6.25
The blockchain.com explorer shows this as having a -6.25 fee (despite having no transactions) https://www.blockchain.com/btc/block/0000000000000000002ba5a1fb96f93e6c215d62db4280f1cbd30e82c7c71fba
5
There is a maximum limit on the block reward which is 12.5 BTC but nothing prevents a miner from claiming less than 12.5 BTC. In fact, there have been times when miners forgot to claim any bitcoin at all (claimed 0 BTC), a very expensive mistake. This is probably a mistake from the miner, he could have certainly claimed more bitcoin.
As long as the block is valid, it will be accepted by the network, even if it's unfair for the miner. In practice, this means that the maximum supply of bitcoin is not 21 million BTC but 21 million BTC - unclaimed BTC by miners. People often see this difference by making the difference between sum(number_of_block_i * block_reward_of_block_i) and the total bitcoin counted from the UTXO set.
3The block reward was 50 until block 210000, when it dropped to 25. Since block 420000 the reward dropped to 12.5. – Pieter Wuille – 2018-09-05T17:13:30.207
1 related: other blocks where miners claimed less: https://bitcoin.stackexchange.com/questions/38994/will-there-be-21-million-bitcoins-eventually/38998#38998
2
Possible duplicate of Can someone explain this weird mining activity?
– MCCCS – 2018-09-05T14:31:09.950While closely related, I think this question is sufficiently different: it's dealing with a different block and someone claiming only half the reward instead of none. – Murch – 2018-09-06T00:15:58.397