I don't recall this having been explained previously, it was just assumed the reader would spot how they correspond. I would like to know too.
The MAC address is already in hexadecimal representation. A digit from the serial number is not - you need to find the ASCII code for the number ("3" in af123's example) and then, for our purposes, show it in hex form - "33". ("Our purposes" being show the contents of the whole key in hexadecimal form.)