Basically, Buck was an amazingly good lead dog after he killed Spitz and then forced the men to put him in as the lead dog.
It was with Buck as the lead dog in the pack that the men were able to set records for the quickest runs. Much of that was due to the fact that Buck was a really great lead dog.
With Buck as lead dog, the team managed to make forty miles every day for a whole two weeks. This was impressive enough to make everyone in Skaguay make a big deal of Perrault and Francois.