The cases are stepped by 60 Ns because that is the amount of one grain. The 100 case is an odd ball and falls between one and two grains. The 360 is a two grain step above the 240 case. The spacers have to work with whole grains, so a spacer won't work with a 100 case reload. Not sure why the one spacer won't work in a 120 case for a 60 case reload, other then it's not certified that way, and it makes for a very heavy motor for only 60 Ns
The table looks incorrect to me. I think the 1 spacer for the 360 case should be DOES NOT WORK and the 2 spacer should be a 29/240 case ONLY WITH EXTENDED SPACER.