Do simultaneous work on the dev
branch. What happens is that in your scenario the feature branch moves forward from the tip of the dev branch, but the dev branch does not change. It's easier to draw as a straight line, because it can be thought of as forward motion. You made it to point A on dev, and from there you simply continued on a parallel path. The two branches have not really diverged.
Now, if you make a commit on dev, before merging, you will again begin at the same commit, A, but now features will go to C and dev to B. This will show the split you are trying to visualize, as the branches have now diverged.
*-----*Dev-------*Feature
Versus
/----*DevB
*-----*DevA
\----*FeatureC