It's definitely not the cube root, either, unless something changed with the quality calculations. It would lower the Q even lower than averaging them would. A low value has a significantly greater impact on a geometric mean than an arithmetic mean, where as an abnormally high value has less impact in a geo mean than in arith means.
math I used sqrt ( (28 * 28 * 10) ^ (1/3) ) /10 ) * 1600 = 2255 approx
to reverse engineer it, assuming the quality scaling is the same--sqrt ( Q / 10 )-- you get approx q23.581441. If I do this: ( (10 + 28) / 2) + 28) / 2 = 23.5
edit: doing some follow up on this, and that math doesn't seem to work on the other listed curios...

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