38º is the real FOV in 1.5X zoom, and the 54.6º is the human eyes FOV below the periscope or binoculars. And at 4x zoom, the human eyes FOV do not change (almost) but the real FOV change to about 9.5º.
The binoculars is used for enlarge the small real FOV to a big human view FOV. If the human view FOV do not enlarge, then binoculars is useless. So what represent in GUI about the periscope and binoculars is the human view FOV.
And they talked about are the TDC auto update "feature". I had some comment on that:
http://www.subsim.com/radioroom/showthread.php?t=140804