I've had a few copywriting assignments I was sure I'd nailed, based on the brief... and the client didn't share my view. Not many, all told, not enough to make me consider a career change, but a few.
It happens. The client may be expecting something very specific (but not tell you what that is), so your response will only ever stun and confuse. Or they may not make the same mental connections you've made between the brief and your copy.
The important thing to remember is that neither party is 'at fault' when this happens. The client may point to you for not understanding the brief. You can point back and suggest they moved the goalposts or just don't know what they want. Neither of you is right or wrong.
There is no such thing as a 'watertight' perfect brief with a single correct response. The same goals can be achieved with any number of (relevant) creative executions.
Take the world of advertising for all the examples you need. Did Cadbury's believe a drumming gorilla was the best way to sell chocolate before an agency suggested it? If there was only one way to convince people not to drink and drive, why have we been making these promos for over 50 years?
Admittedly, the brief given isn't the most comprehensive one ever written. And Radix deliberately picked a range of writers with different backgrounds. There's no way that didn't influence the outcome.
That said, the results are an interesting read for any writer. I bet another 8 copywriters could produce 8 more totally different responses. Give it a look and see what you reckon.