WingFlaps writes:

You need to understand the vertical relationship between airspaces.
Have a look at a sectional and think about it. Why would you choose to
fly on the vertical limit between to airspsaces anyway?

I don't know, but if the charts didn't use the + and - notation, you'd be able
to, and you'd have a defense for doing so if you were called on it. Clearly,
the FAA doesn't want to leave that loophole open.