That is relative to "itself" only. Including frame 0.
If the bone is rotated at frame 0 and you get "bone.fAngle" at frame 1 it will be the same value as frame 0.
However, if the parent is rotated let's say... 90 degrees and the child is rotated 0 degrees it looks like it is rotated 90 because it is in a straight line from the parent, but it's really not rotated at all.
Often I find that child bones are rotated 360 degrees... when they SHOULD just be 0. Or I find that a child is rotated 270 degrees when it should be -90 degrees. It has to do with how and when they are parented...
... or maybe that crazy "radians" thing is playing tricks on me.
I suppose this might require another script like "RotateRotate".
-vern