The speculation is that it's not worth the effort of redoing the Spells tab just to remove Garrison, Clairvoyance, and whatever else is there, since the new client is coming out relatively soon.
Dominion returning in the rotating mode queue wouldn't explain it either. Dom's been downgraded to a Featured Mode by Riot, and other summoner spells for Featured Modes (e.g. Poro Toss) haven't been shown on that tab.
As an aside, if you're trying to point something out in the future, be specific. Vague hints (such as "you'll understand after you look at the tool tips") just make it more annoying to figure out what you're talking about.