Your total compensation at FAANG is base salary + bonus (usually % of your base multiplied by your perf and company perf) + stock.
When you get promotion your base salary will increase, your bonus will also increase (due to being % of base, but also higher levels have higher %) and your annual stock refresher will be higher, as you are now on new level.
She would get annual stock refresher even without promotion, but with promotion refresher would be bigger and total new comp could have compensated for drop after initial grant has finished.
You can check levels.fyi for comparisons of total median comps between levels at Google.
I guess I am trying to understand whether this is causation or correlation relationship.