Okay, here is my observation.
These are the three possibilities of the lume application for rep dials while the first (over sausaged) is commonly seen. You can see from the pics posted by thatoneguy that the the dial probably has a very shallow 'space' for the lume to be applied on and usually they are 'messy' as edges are not fully filled. Therefore, these empty edges create a fake impression of 'cavity' or 'recessed dial' that we speak of. Another fake illusion is created when a lume is slightly higher than the dial, therefore casting the shadow outside of the lume thus the impression of recessed dial. It's hard to explain in words but these are my opinions from studying my 411 dial under the loupe.
The effect you see on gen dial is a result of a perfect and consistent lume application on the lume space (or whatever you call it). The edges of the lume space is slightly curved, just like the illustration on ale's diagram. I've studied a Pam005 and 000 upclose and the difference is huge in real life. With gens, you see a really nice consistent recessed effect but with my 411 rep, its more like a drain surrounding the lume lol. Anyway without a gen beside, one can be tricked into thinking that its a gen dial.
but very shallow recess effect but enough to show the gen like appearance 
