With a sharpening jig, you clamp the blade near the middle, place the guide rod of the stone in the appropriate slot to set the angle and begin running the stone over the edge as you gradually move the stone from end to end of the blade. If you examine the geometry involved you'll realize that the angle will change as you move out toward the ends of the blade. As the stone moves farther from the center of the blade the angle will decrease. Again, if you lengthen a leg of a triangle, its angle will decrease.
The diagram illustrates how the distance from angle slot to edge can easily double as the stone moves to the end of the blade. This will make the angle continuously variable from a maximum at center and gradually decreasing toward the ends of the blade.
If you do the math (uh, the geometry) you'll see that the change in angle is very small, especially for a short blade. If you're sharpening a 3" pocket knife, you won't be able to measure the difference. But what if you're working on an 8" chef's knife? The angle can change a degree or two or even three from center to end of a long blade.
OK, but the real question is, "Does it matter?"
Answer - No.
If you surf the Web for information on knife sharpening, mosts sites you find will tell you the normally used range of angles for different types of blades. For example, most experts advise a relief of 5-10 degrees and an edge of 7-15 degrees for a hollow ground blade. That's a pretty good range - lots of elbow room so to speak.
So here, again, the point is not the absolute angle, it's repeatability so that you can bring back a fine edge on a blade while removing a minimum of metal. Using the same jig in the same way will result in the same variable angle each time which will result in minimum metal removal.
More than you ever wanted to know about edges and sharpening.