The expected difference between the most common rune and least common rune in a group is roughly:
0.5*sqrt(total_rune)
Assuming the drops are TRUE random (as opposed to pseudo random)

So if your group have 30 players generating 10 runes each for the last 7 days, that's roughly 22.
The 0.5 bit is my estimate, it's probably some constant between 0.2 and 1.2, but I can't think up the proper formula off the top of my head, it is a matter of standard deviation but... well.... anyway... you got the picture. There is nothing really wrong with it, it's just randomness at work.
a year from now, it's going to get about 5 times worse.