That is irrelevant to the discussion at hand.
But most likely the reason is that it cost less in the purchaser's eyes to do it this way than to buy a discount Biz fare that cost more.
John
It is only relevant because the OP just stated that he did not buy coach because he did not want them to sit in coach. I like upgrades as much as the next guy but if I really want to know I am in biz I buy biz. That would have solved this issue so I do think it is relevant.
Edit to add: "it was cheaper this way" = that is why it is cheaper. Upgrades are not guaranteed and that is why it is cheaper than biz.