The two important concepts to understand when discussing SSL performance are "TPS" or Transactions per Second, and bulk encryption. SSL TPS is the initial SSL handshake and key negotiation (and to some degree re-negotiations). The size of the keys are important here because they are used in the initial session seed key encryption and digital signing. After the handshake, both peers have a (generally smaller) session encryption key that they use for bulk encryption. The bulk encryption number is therefore always larger because it's using smaller keys. The difference in SSL TPS and bulk encryption numbers across platforms is based on the SSL ASIC (SSL offload chip) processing ability, and to some degree system processing. Anything less than the "max" number would be a licensing thing.
Further, the difference between 1K and 2K key sizes and TPS has to do with the cryptographic intensity of each, where 2K keys are more complex than 1K keys by a factor of about 5.