6970 is an ultra-stuffed card that was pushing the limit of the 40nm node. 7970 is brand new design, and it's a simpler design due to the 28nm nodes being so new and relatively immature. It's like the 5870 before, which was also a new entry at the then-current 40nm node, and could overclock to then-unheard-of levels.

Just look at it here:
http://www.guru3d.com/article/radeon-hd-7970-overclock-guide/1
The default settings in CCC allow for a 200mhz speed bump (from 925 to 1125). That's default voltage, default everything.

Add to that more efficient coolers that'll happen with 3rd party vendors, better software support in overclocking apps like afterburner, maybe even factory overclocked models, and we got ourselves a monster.