Yes, photos don't always show what you see in person. Here's a pair of juveniles at an age a bit younger than yours. Photo taken in the bright sunlight.
Here they are as adults:
At any rate, if the odd colored chick is from the same parent stock as the ones that look like FBCM, then there are some odd genetics in that particular flock. Or, maybe a rooster got into the wrong pen. It can happen.
No, that is not a black tailed buff marans. BTB chicks are more golden than yellow at hatch, and a pure BTB wouldn't be getting black on its back and chest. I would guess it's a wheaten or a sport out of a BTB project so might not have true wheaten genetics depending on what the breeder is...