There are not 21 000 000 bitcoin (although this is a nice rounded number to indicate the fixed max supply).
The real number is lower.
Block 124724 tried to intentionally claim 0.00000001 BTC less than allowed, but accidentally also failed to claim the fees, losing 0.01000001 BTC.
Between block 162705 and block 169899, 193 blocks claimed less than allowed due to a bug, resulting in a total loss of 9.66184623 BTC.
Between block 180324 and block 249185, another 836 blocks claimed less than allowed, resulting in a total loss of 0.52584193 BTC.
Block 501726 had no transaction outputs (except a 0-value commitment), losing the entire 12.5 BTC subsidy.
Block 526591 didn't claim half of the block reward, losing 6.25 BTC.