How does the G2/G3 R command work?

[EdChamberlain]

Ive been playing around with G2 / G3 arc commands and I've hit an odd issue. Ive been using the command:

G3 X100.0 Y150.0 R50.0 E0.0

to try and draw an arc from from (X150.0 Y100.0) to (X100.0 Y150.0) in a circle with radius 50. In my understanding it should rotate the head around the centre point 100, 100 but in practise the centre point of rotation is off. It seems to make the circle smaller and move the centre point towards the arc segment.

If I replace the g2 command with:

G3 X100.0 Y150.0 I-50 J0 E0.0

it works perfectly.

Am I missing something here?