How dose this work

Funny little link I found can you tell me how it works.

http://www.regiftable.com/RegiftingRobinPopUp.html

fdd posted this samething a little while back

basically the math always leads to specific numbers and all those numbers have the same answer.

example: all the numbers from 20-29 will reduced to 18 and all the numbers from 30-39 will reduce to 27. notice how the answers in those to spots are the same?

Thank you Kant.