This information can easily be calculated by the number of blocks in the chain - as 50 BTC per block were created for the first 210 000 blocks, then 25 BTC per block for the next 210 000 blocks, and so goes. Look below:
Blocks 279383
Total BTC 12.235M
Starting from 279 383 blocks you can find:
210 000 * 50 = 10 500 000
(279 383 - 210 000) * 25 = 1 734 575
10 5000 + 1 734 575 = 12 234 575
12 234 575 which that site rounded up as "12.235M"
This is not an exact value but a close enough approximation and it seems to be what most sites are using.