Are you sure full was actually full? The one time I filled my FW it seem way too soon that water came out the overflow. So I gave it a minute to settle, started the water up again (a little slower by only opening the valve 1/2 way, then up to full once it was running good) and probably put a good 2-3 times more water in the tank. Spent 2 days camping with 2 toilet training toddlers, who don't know how to conserve water yet, throw in a bath for the two of them, and still had 1/2 tank left when we went home. I'm sure if I had stopped when I first got water out of the overflow, I would have been out of water by lunch time on Saturday.
There's no use crying over spilled milk, unless it's on your keyboard.