Administrator 10% does what?

I assume it's the speed at which things are constructed.  Increased production equals reduced build time.

I'm not sure yet but I am of the opinion that it either does as Mtn_Man said or it is like a farmer trait and increases material production by 10% for the city the hero with that trait is located.

I once had 5 administrators in one city and everything was built in half the time...then I cast that +25% spell and everything was getting built in 1/4 the normal time.

It affects construction time. When you open the city details screen you can see the current ratio there. Normally cities would have 1.0 so buildings would take 100% of the base turns to complete. With an administrator it would go down to 0.9 so only 90% of the turns would be needed. With a couple of administrators, the kindgom buildings that reduce construction time and the enchanted hammer spell you can easily reduce construction time to 0.1. That's when I first noticed there is a cap of 50 tiles per city.  So I just moved all my administrators, recast the spell on my next biggest city... 

i think it is funny you only noticed the tile cap when you had ideal building conditions 

Probably because I started playing close to 1.1 beta and I kept starting new games with each release.