If your serialization expects a 65byte signature and you only provide 63 byte .. then it needs to either perform running length encoding or do zero padding. The former leads to additional data on the serialization while the latter leaves you with meaningless bytes in the serialization that can be used to hide information (e.g. leakage of private key data) -- this is particularilly a concern for hardware wallets where you don't see what code is actually executed.

That said, restricting signatures to 65bytes is easiest with respect to security, scalability and network efficiency.