Payment help: a different amount of money came off buyers account than buy price

Payment help: a different amount of money came off buyers account than sellers buy now price.

A seller on my auction theme website sold something for £10.

When the buyer clicked “buy now” and confirmed to make the payment through paypal, only £8.50 was sent to the seller, not £10.

So..
I (website owner) received £1.50 (the 15% fee) in my paypal business account from the sellers paypal, which is fine.

But why has the seller still only received £8.50 from the buyer, she should have received £10.

So why was only £8.50 paid when “buy now” was clicked?

I have set my paypal adaptive settings to only keep 15%, the house fee is set to 0%.

Why is it doing this and How do I fix this?