Spell Variance in Shadowlands Update: Ion provided some additional context.
Originally Posted by Blizzard
(Blue Tracker / Official Forums)
In an upcoming build, most non-periodic abilities on all classes will have a small amount of variance on the damage or healing of each individual use.
This was standard behavior in WoW for many years (and has its roots in familiar mechanics from many RPGs), but recently was lost as a side effect of some under-the-hood mechanical changes. We’re restoring this behavior now to bring back the small bit of texture, and avoid the result where using the same spell repeatedly results in exactly the same 3- or 4-digit number every time.
We’re keeping the amount of variance small (5% currently), so that the impact on total performance, over the course of a combat with many events, is negligible.