G90 is absolute and G91.1 is incremental arcs.
But the way cambam is doing it is wrong.
Run this code, and watch the mode in the status bar. It will never change to G91. And if you were in G91 mode when you started, it won't change to G90.
G90 G91.1
G4 P2
G91 G90.1
G4 P2
G90 G91.1
G4 P2
G91 G90.1