Why do certain damage buffs not affect the visible DPS number on skills?

The one that's eating me right now is my ranger has 2 quivers, each with almost identical stats, but one has +2-40 lightning damage and the other has +38% projectile speed, which on my ranger is also a buff to bow damage because of the passive skill I have. But when I swap between these 2 quivers with my skill tab open it shows that my skills are doing almost 700 more DPS with the one with no projectile speed.

Ive tried testing them but with crits and other Random buffs that can happen they seem almost identical. I'm just frustrated that I can't rely on the DPS the game tells me a skill has because I might have a passive that affects my damage output.