Each facet on a graver has what I'll call for the moment a pitch angle, and a roll angle. The pitch is how far it is tilted up, and the roll is how far it is tilted to the side.
The face of the graver for instance, that part which is mashed into the metal out front, has a pitch angle typically of 45 to 50 degrees (0 degrees being parallel with the axis of the graver and 90 degrees being a right angle, as if it were just sawed off square). It has a roll angle typically of 0 degrees (not tilted to either side).
Now if you look at the graver from the front, lined up with it, you'll see the bottom of the type of graver you are looking at looking like a V. The angle of that V determines the width of the cut compared to its depth. The narrower the V (or the sharper if you prefer) the deeper it will have to go to give the same width cut. The wider the V, the shallower it would need to cut for that width cut in the work surface.
130 degrees is probably the included angle of that V. It keeps you from having to plunge the graver too deeply into the work surface to get the width of the nicks. The bottom facets, forming that V, are rolled 25 degrees each way. 25 + 25 + 130 = 180 degrees.
I hope this helps some! It is easier with pictures.
Gerald