DPS comes from piecewise stat weights, not a full simulation - hit is weighted heavily up to the cap, then at nothing for a mage and about half for a warrior or rogue, and set bonuses and proc trinkets are flat additions.
Weights shadow_priest/v1-launch · content vanilla/20260828-112322. These weights come from published community values for the spec, not from a simulator fit. The ratios are credible, so the choice between two items is the right question - but the DPS number is internally consistent rather than a real Vanilla DPS figure.