The problem is that (according to this paragraph), if you're playing a half caster, you add them together before you round down, whereas if you're playing a third caster, it assumes that they're played in isolation, so you're only invited to tally one or the other before rounding down.this seems to read that the 3 groups are added together independently before dividing by the appropriate number. .
At worst they're trying to nerf MCing with third casters, but the much more likely explanation is that it's just a poorly worded paragraph.