I've had a Lennox HVAC system installed in a new home in Central Florida. The equipment is:

A/C Compressor - Lennox XC21-060
Coil - Lennox CX34-62D-6F
Furnace - Lennox G60UHV-60D-135

The Lennox literature and website advertise "up to 20.5 SEER" for the XC21. I realized that the "up to" meant that it might not be quite 20.5 based on the coil / air handler, but did not expect that the rating tag added on installation would say it is a 16.5 SEER. That's about a 20% reduction from the advertised number.

Is this a normal thing? What equipment changes would it take to get this close to the advertised 20.5 SEER?