Actually, they don't. The A variants have middling Endurance, a miniscule amount of secondary stat, and a lot of primary stat. The actual itemization budget for DPS attributes is worse than the unlettered variant and only slightly better than B variants. It's for this exact reason that pretty much everyone outright prefers the unlettered mods (some tanks like the B variants for the higher Endurance but not all; all DPS and healers should prefer unlettered mods).
Hmm, well that's embarrassing. I'm not sure how I thought otherwise, but the itemization is right there. The A mods actually have *more* endurance than the unlettered mods, and the main stat contribution absolutely doesn't make up for the power loss (unless you're one of the weird classes where main stat has an INSANELY high weighting).