@leusee No, I actually googled around first and the "cup per foot" was the correct number - for possible, i.e. potential perspiration.
Like you say, a normal person on a normal day will perspire less, so the feet will also perspire less than the maximum potential.
But on the other hand, feet are usually the "most dressed" part of the body, in that you might have thick sneakers even when the rest of the body is fairly lightly dressed. So the feet will comparatively sweat more.
(Also, some deoderants for your armpits are actually antiperspirants that will stop your glands from being able to release sweat. If the "normal numbers" include that, then feet will also sweat comparatively more)
(Also, and I haven't double checked these numbers in any way, but it seems like 20% of your sweat glands might be on your two feet)