In PVP, spells can be altered by a PVP multiplier which isn't documented anywhere in the official UI.
UPDATE: I've decided to throw in the towel since 8.2 patch. BFA PVP is just disgustingly bad. This addon is now abandoned and won't be updated anymore.
PVP Tooltips is an AddOn that displays these multipliers on tooltips.
- It is compatible with all languages (English, French, German, Italian, Korean, Spanish, Russian, Chinese and Portuguese).
- Compatibility with other AddOns that modify spell tooltips is not guaranteed.
Hopefully, Blizzard will support PVP multipliers on standard tooltips in the future. In the meantime, PVP Tooltips will document PVP multipliers the best it can.
In instanced PVP zones (arenas and battlegrounds), PVP Tooltips also displays stat templates multipliers for your character's specialization on the Principles of War aura tooltip. (removed in BFA)
Note: Blizzard may adjust PVP multipliers on spells throughout PVP seasons, it's very likely that this AddOn will need to be frequently updated. Due to a fairly lacking documentation from Blizzard's part, PVPTooltips may list inaccurate PVP modifiers on some spells.
Tip: press ALT key before hovering your mouse cursor on a tooltip to toggle between PVP modified version and regular version of the spell (only when outside of PVP).