10' wide hallways aren't always 10 feet wide.

This has been bothering me recently.  Look at any old school map, like the example below:

You would think that each of the halls here are 10' wide if you assume each square=10'.  But unless those walls are extremely thin, the hallways are probably closer to 8' wide.  That's not to say that the 10' wide hallway doesn't exist; any situation where there isn't another hallway or room on the other side of the wall would allow for them.

Does this affect gameplay?  Should I use these new hallway sizes when calculating the volume taken up by my fireball spell?  No.  It's just a pet peeve of mine.