I think a lot of it has to do with individual biology and mouth chemistry. Some dogs (and some cats and humans) have a naturally more acidic oral environment than others. Thus the same foods and residues on the teeth can be more problematic in these individuals than they would be in others.
Also, if your dog has any other oral problems (crooked or missing teeth or the like), this could create the opportunity for more plaque to build up. Or simply if the dog is a different breed; some mouth shapes may promote plaque accumulation more than others.
Eating habits could also potentially play a role. Not just what the dog eats, but how it's eaten, when and how much water is ingested, when and how bones are chewed on, etc. I would say that mouth pH and dental layout/mouth shape are much more likely to play a role here than eating habits, but it still could be a possibility.
As far as improving the situation goes... it may just come down to a more intensive oral care regimen for this particular dog. I have seen some reports that very large kibble can have a beneficial effect on plaque build-up, but I have no direct experience with that. You could try giving him additional bones or dental chews, but you may also just need to clean his teeth more frequently than the other dogs'.