Her face may fluctuate greatly. What does it mean?

It means that there may be great changes.

Floating means that the face may be unstable.

Floating refers to the behavior with little change, that is to say, floating refers to little change, or no big change. Floating can be slightly upward or slightly downward. Drifting refers to the behavior of drifting on the river due to the influence of water flow or strong wind, and the range of drifting can be large or small.