How does this work?

Alright I can't figure out how this program keeps guessing correctly, it must be a math trick:
http://www.regiftable.com/RegiftingRobinPopUp.html

there was a thread awhile back like this... i think it every time u do it ur answer is divisible by 3... and everything divisible by 3 has the same answer ??? idk think thts wat it was