I didn't notice that :/
I like my classification better-they seem to fall naturally into the four allomantric quadrants.
I am of course prejudiced.
Three also keeps showing up because each Shardpair has three possible arts: one positive (Allomancy, Aon Dor, Surgebinding (?)) one negative (The Dakhor priest's arts, Hemalurgy) and one balanced (Feruchemy, the Jindo dance).
That is what I was thinking