The custom CRC feature calculates 0 all the time until you press the "Custom CRC..." button first. The values that are saved in there are saved and will be set right already, so you can immediately click OK. But if you don't do that the CRC will calculate to eight zeros. Custom checksum appears to do something similar.
Looking at it more, the custom CRC line in the available algorithms also shows 32 bit, until I open the screen, then it changes to 16 bit as that is what I have selected.