Nigel, I’ve never seen an article here that refers to the expense split you mention, and a full search of the site hasn’t turned up any reference to a split of expenses according to the law, or “expenses arising from the public deed, ‘by law’”. I can only think you must have read about this at another website, unless you can show me a link to where you read this?
There are a bunch of expenses related to buying and selling property in Spain, such as notary fees, registry fees, and taxes, but to the best of my knowledge the law is pretty clear which side pays which cost, though both sides can always agree to a different payment structure.