Spanish Will Question

The deceased had assets in the UK and Spain but was UK tax resident and UK domicile so all worldwide assets included in his UK IHT 400. His UK will included an exclusion clause for any assets held in Spain. He had a Spanish will to deal with the Spanish assets and probate has been obtained in Spain. On applying for a UK grant the UK will was submitted which prompted a request to submit a copy of any Spanish will together with a translation. Whilst this has been sent can anyone enlighten me as to why the UK probate registry require this document?

Thanks in advance.