Before you accuse the numbers of being tweaked, go back and ready the way I came to the numbers.
Nips, the final numbers I had posted for damage increased were based on special rotations for each weapon style and spec (the numbers that included both auto-attack swings and specials). The reason Frost dual wielding came out lower is because the frost dual wielder will use IT, HB, and FS as most of their rotation, even including the OB/DS's filled in when Rime procs left them with fewer parryable attacks than Unholy single target where they're using BS, ScS, PS for most of their swings, all of which are parryable. There are debatable assumptions, but that's really not one of them.
Fonager, if you're tanking as a Frost 2-hander, the question is, at what point do you decide it's better to do HB than Oblit for the sake of limiting parry-haste. Personally, I often favor HB anyway, even if Oblit may hit a bit harder (which isn't always the case, that depends on spec, gear, state of debuffs, and target properties like armor). Also, with my frost rotation I deliberately use IT a lot, and only PS to keep the disease up. Extra IT's, which hit plenty hard on their own, also give more chances for free HBs. Generally speaking, Frost has a lot more mechanisms to avoid parry liability regardless of weapon choice.
The fact that we can use 2-handers drops our parry liability in general WAY down, and the only thing that brings us back up on par with other classes is our specials. The numbers reflected above also take into account RS. For those who aren't familiar, RS is not an in-addition next swing move, it replaces our next melee swing, and is unparryable (or dodgeable, or blockable). The major reason that dual wielding "could" come out on par or better with a smart frost rotation, is because you can make use of more Rune Strikes, reducing your auto-attack liability a fair amount so it's not THAT much worse than having a 2-hander.
The numbers aren't slanted, and they weren't back-engineered to favor dual wielding.