I think this may have been a case of that specific sword simply having wildly inflated secondaries. If I recall correctly, when I played during Wrath, +STR and +Hit were the stat kings for plate DPS classes (I remember all the guides recommending +STR gems in every slot and +STR enchants on everything) and secondary stats were, well, secondary, and inflated secondaries on a weapon weren't that big a deal. But now that secondary stats have so much more importance, I suspect that an "outlier" weapon like that just wreaks havoc with modern calculations, so that all the addon sees is that this particular sword offers +15 more Haste and Crit than everything else that drops.
Last I wanna share a curious bug that almost freezes my game and force me to disable the addon. Anytime I add a (Citadel Crossbow [of the Fireflash]) my fps go from 75 to 22, and It's on a paladin, paladins dont even use crossbows. I noticed everytime I put something in my bags the addon takes the math and my fps goes from 75 to ~22 for a while (3 to 7 seconds) and is think it's internal and also think it's fine.
Heirlooms are special pieces of armor that level up with your character, so they have stats that are always respectable for your character’s level (up to level 110). Heirlooms that fit the head, shoulder, chest, legs, and back armor slot offer an experience bonus. The total bonus is 45 percent if you have them all. It’s also possible to acquire a ring that adds another 5 percent, but you must win a fishing contest acquire it, and that can take some time.