It's not just that one is twice as old.
Or that Bjorn is a 40-year-old pro who wonders how much longer he can compete at the highest level, while Lewis is an amateur making his major championship debut, his best golf still to come.
The biggest difference are their memories of Royal St. George's.
Bjorn took a small step toward atonement with a birdie on the par-3 16th -- the hole that cost him the claret jug in 2003 when he took three shots to escape a pot bunker -- on his way to 5-under-par 65 in the toughest conditions of the opening round.
He made a birdie on Thursday, and couldn't help but smile when he saw it bounce away from trouble and toward the flag.
"When I hit the shot, I thought, 'This is going to struggle.' So when it just made it over that bunker, that was just a smile of knowing that things were going my way today," Bjorn said.
Lewis ran off four consecutive birdies late in his round, an amazing stretch that began on the par-5 14th. That's the hole where Lewis wrapped up the British Boys Amateur Championship two years ago, the highlight of a sterling amateur record. A par on the final hole gave him 65, the lowest ever by an amateur in the British Open, making him the first amateur to lead this championship in 43 years.
"It was a special moment for me, winning here, and to come back to where you've won is extra special," Lewis said. "I was just thrilled to be here, but to shoot 65 the first round was something I wouldn't have thought. I was just happy to get the drive off the tee at the first, and that was all that mattered."
Adding to the nerves was playing alongside Tom Watson, such a popular figure in the Lewis household that they named their oldest son after the five-time Open champion. And to think the kid only wanted to make sure he didn't embarrass himself in front of Watson.
"He could be my grandson," Watson said. "I just had to smile inside to watch him play. I didn't play particularly well myself (he had 72), but I certainly was impressed by the way he played."
Bjorn wasn't even in the tournament until Vijay Singh withdrew on Monday.
Miguel Angel Jimenez had a bogey-free 66. He was joined later in the round by former U.S. Open champion and ex-Clemson star Lucas Glover and Webb Simpson.