Smart watch notifications - why does pressing the 'down' clear button the message?

After receiving a notification from my phone on my watch I can press the 'down' button to clear it.

That seem odd, why doesn't pressing the 'down' button display the rest of the message? Only the 'back' button should clear the message.

It seem really counterintuitive that the 'start' button displays the details of the message instead of the 'down; button.