Because in that example steem was valued at $2.00 and he was describing 50/50 reward selection - 50% steem power and 50% steem dollars.

The steem power you would recieve gets cut in half in this example because its valued at twice as much as steem dollars. If it was valued at $1 you would have recieved the 5/5 but at $2 you get half as much.

Steem dollars are valued at $1 so you would get the full 5.

This can be verified on your wallet page.

STEEM DOLLARS
Tokens worth about $1.00 of STEEM, currently collecting 0% APR