If you look at the rotation values in the graph editor, you’ll see that the lower layer is actually 5 seconds behind the upper layer.
Just a couple of notes on your expressions. You don’t need the final variable, and you don’t need to put the result in brackets (because rotation is a one-dimensional property), so this would do for the top expression:
initialRotation = 0;
rotationIncrement = 10;
initialRotation + rotationIncrement + Math.floor(time)
and this for lower one:
R = thisComp.layer(index-1).transform.rotation;
R.valueAtTime(time-5);
Also, I’m wondering if the upper rotation is actually what you meant to do or whether you needed a multiplication, like this:
initialRotation = 0;
rotationIncrement = 10;
initialRotation + rotationIncrement * Math.floor(time)