What does it mean to have "signature verification with RSA-4096" if the key is only 3072 bits long?
Two possible interpretations:


"The RSA key sizes supported are 1024, 2048, and 3072 bits" could mean that RSA key generation and/or private-key operations (signature generation, perhaps decryption) is supported only for 1024, 2048, and 3072-bit key (really, public modulus) size; while "up to RSA-4096 signature verification" would apply only to signature verification, or more generally public-key operations (also including encryption).
The restriction to 1024, 2048, and 3072 bits could apply only in the so-called "FIPS mode", because FIPS 186-4 section 5.1 and by extension FIPS 140-2 conformance only allows these particular key sizes when using RSA.