First, there are no keys that are encrypted with AES256. The material that is used for the cryptographic functions are negotiated.
For the difference 192 and 256: For WPA3 Enterprise there is a 192Bit-mode. I assume that it's what you are reffering to. This mode provides a 192Bit security-level, but uses AES-256 for the encryption. Encryption is not all that is done in a cryptographic system. There are other functions that are "only" on a 192 Bit level which is the reason it's named "192 Bit mode". The encryption is used with 256 Bit to make it resistant against coming (sooner or later) attacks with quantum computers.