What they mean by "change the damage calculations of tanks"?

[Alahra]

Healers who want to pull more dps aways have been geared with accuracy melded gear or vit+ melded gear to survive hard progression. DPS are the only ones that used only non-melded tome/raid gear.

DPS used VIT-melds at various points in early progression as well throughout ARR, so they could hit various VIT-checks, though they often only needed one accessory, perhaps two. It's something the developers weren't fond of, though, which is why they started delaying the release of crafted accessories.