Imagine a moment froxen in time, as below. At this instant the FM reference signal is at a phase of 0º, and going out in all directions. The AM signal has a maximum amplitude at a bearing of 270ºM and a minimum at 090ºM
The point of maximum amplitude on an AM transmission corresponds to a phase of 90º, the point of minimum amplitude is a phase of 270º. This means that, at a magnetic bearing of 270 from the beacon we have the FM at 0º phase and the AM at 90º. The phase difference could be read as 90º or 270º but, by convention, it is read as 270º.