This is interesting. This is the response from Paypal Merchant Technical Support:
Thank you for contacting PayPal Merchant Technical Services.
The Sandbox is designed to easily allow merchants to test their website and scripts without real money changing hands. As part of this Sandbox does not auto-decline mismatched credit card information like the live site.
What is the point of 'registering' the credit card info and name on the Paypal Sandbox Seller account then?