I was thinking of this from my point of view as a "newish" player reading the book: you'd get to page 18 and say "OK, you had me roll characteristics as a foundation for the rest of my character, now you're going to give me some bonuses based on what I rolled..that's cool, I like bonuses!....hold on a second ...now you're going to give the same bonus to my buddy who rolled way less than I did." Basically I don't like that you say "roll these dice to get a numerical value for a characteristic....wow, you got an 18...that's the best you can get!...now here's a bonus based on those rolls....we're going to give you the same as if you rolled a 13, is that OK? In other words, we're going to not reward you for being the best, instead we're just going to lump you in with the top 3rd of your class."
I think some of those tables are setup because someone like the symmetry of having them grouped by 6's instead of some other grouping. That's fine for most people I'm sure...except for people like me that think if you roll much higher than your buddy, your bonus should be much higher than your buddy.
And I also realize with the right GM and group it won't matter about relatively minor nitpicks in the rules (which I suppose it true of any game not just Runequest). But, I'm reading right now not playing so these kinds of things "stand out.". Again, with more experience I'm sure these things won't matter in the grand scheme of things, but on the initial read it was an "issue"....but of course we'd just use our own homebrew tables that we like better, so it doesn't really matter.