Hedge Ratio - Currency Futures

Per CFAI: “[If] the hedge ratio is assumed to equal 1, the changes in futures and spot prices will be equal during the life of the futures contract, and so the hedge will be fully effective.”

The hedge ratio for futures can be calculated as:

HR = amount of currency to be exchanged / futures contract size

How does this make any sense? Why can I not have hedge ratio of any number and perfectly hedge? So lost, please help