Aircraft call signs changed by tower?

Hi,

I've noticed when a Piper Archer or Warrior calls up the airport tower
at my local airport, they use "Archer 1234" or "Warrior 5678". The
tower then responds with the the call sign used during the initial call.

However, in almost all cases, when I hear a Skylane or Skyhawk call up
the tower, "Skylane 1234" or "Skyhawk 5678", the tower changes their
response to "Cessna 1234" or "Cessna 5678".

Why change Skylane and Skyhawk to Cessna, but not Piper in the former
case?

Thanks,
Joe

-- Joe Moreno, March 10, 2011