In The Book Thief, why does Liesel find Rosa's reaction to Max unusual?

Liesel finds Rosa's reaction strange because rather than the anger she expects.... Rosa looks worried.

Curiosity got the better of her then, as she imagined a tirade thrown down from the wrath of Rosa. There was definite movement and the shuffle of a chair.
After ten minutes of excruciating discipline, Liesel made her way to the corridor, and what she saw truly amazed her, because Rosa Hubermann was at Max Vandenburg’s shoulder, watching him gulp down her infamous pea soup. Candlelight was standing at the table. It did not waver.
Mama was grave.
Her plump figure glowed with worry.