It looks like credit = runtime / 432.
This means that faster machine will get less credit than slower machine for the same amount of work.
Even more, the run time is wrong. It counts run time from last checkpoint only. In reality first 8 tasks on my screenshot ran for about 56 hours, but this computer wasn't powered on continuosly.

Hi! When granted credit is proportional to amount of calculations, for each computer granted credit = run_time / k. But k are different for different computers. :)