ADAT channels are paired: (CHannel 1/2, CHannel 3/4, CHannel 5/6, CHannel 7/8), by default as the L/R channels of a stereo signal.

So ... unlike on a DJ mixer, for instance, where the L and R of a channel are separate but you still only speak of Channel 1 ... when referring to them, you speak of them as individual channels for clarity's sake, because, whilst they are paired by default, there are eight of them that could be used individually, by panning a mono signal hard L or R.

Hence, when speaking of them in stereo pairs, you still mention them as separate channels, because that's, technically, what they are — trying to describe them in standard (home) stereo terms would be confusing, because it wouldn't be clear to someone else what you were talking about, if you started talking about Channel 1 L/R, because there's no way to speak of that configuration.

Hence the 'fractions' 😀

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You wouldn't like being a DJ, I don't imagine — you need to know your 7x table off by heart, so that you can calculate bpm by 0.7%ths (yes, %ths 😉)