Makes sense. Much of Kepler's strength comes from GPU boost. In multi-card configurations, load needs to be balanced correctly or you get stuttering waiting for one card to catch up. Odds are GPU boost doesn't work quite the same in SLI since no two cards will clock the same with power usage.
Another problem with this is that on air, the inside card will be hotter than the outside card, lowering its ability to benefit from GPU boost. So essentially what we are seeing is the "no GPU boost" cards in SLI, compared to the single card.
Looks like good SLI setups will require cards that are water cooled and have comparable OC headroom.
I think a lot of people are missing the fact that at stock speeds, odds are the 680 is equal to or slower than the 7970. The GPU boost is what gives it the single card performance crown.