When public key is the first (original) key (also known as matser key), extended public key is the extended public key derived from this master key. No matter how many extended public keys are derived from master key, their corresponding contents are identical.
The purpose of extended public key is that when the node (node) is getting deeper and deeper, it can be checked directly with extended public key derived from the current node without going back to the fundamental node step by step to verify the public key.