It's because the buyer is offering what they are willing to pay in total, which, although they might not know it at the time, includes the BPF and whatever the seller will get.
Ebay is then deducting the BBF so that the seller can see what they would get if they accepted the offer. That all seems very clear to me.
If they didn't take the BPF until you had accepted the offer you, as the seller, would get a nasty surprise.
