There are exceptions in every breed. I've known a couple of calm BCs . . . one was an agility champion, but his owner was a professional handler and looked all over the country to find him. He was a "switch" dog - calm as could be until you went to the line.
Jack (or Jac-Jac, as my granddaughter calls him) is like that. When he goes, he GOES. But when you come inside, he curls up next to or on top of your feet, and is as happy as can be - provided he is close to his people. Our vet, a dog sports fan, claims she has never seen such a relaxed Border Collie.
Our previous Border Collie was also like that. They both came from working lines, and that may have something to do with it - a hyperactive sheep dog dies shortly after harassing the sheep...
BTW - the pound mutt is an awesome guard dog. Extremely gentle, but very alert.
In his own way, he’s every bit as smart as the Border Collie. He figures it is HIS job to alert, and MY job to get a gun and investigate!