In other words, the Mossie could theoretically carry 8,000 lbs. extra weight, but could only handle at most a 4,000 lb. bomb, due to drag and bomb size. And in kind, the B-17 could probably carry more than 8,000 lbs, but it's bomb bay wasn't large enough to hold more, and it wasn't equipped to carry bombs externally (AFAIK), Plus, you wouldn't take a full bomb load on every mission, 6,000 lbs. is more reasonable. Get's the job done almost as well as 8,000 lbs of bombs, and saves on fuel efficiency
Following suit, the Lanc could probably theoretically carry more than it's max bombload, but the figures were most likely closer due to the large bomb-bay