Snap to grid not working with Ctrl key

I’ve been following a tutorial to create a building, well actually it’s from a book. If I create a new plane then go into edit mode I can extrude one of the edges and hold the Ctrl key to constrain my mouse movements to the grid. That’s great and what I expect to happen.

However, if I add a loop subdivide (Ctrl+R) and then attempt to constrain its position with the Ctrl key the new loop doesn’t seem to constrain to every grid line, it seems to jump by several. I also find if I zoom in nothing seems to constrain to the grid correctly.

Is this a bug/feature or am I doing something wrong?

You can also hold down the SHIFT key with the CTRL key so snap to smaller units.