What typically happens, and often under the exact conditions you
describe cold humid morning with frost on the ground is that as you
taxi out ice builds up in the carb reducing the RPM. You wil
subconsously keep adding power to keep it running like you want/need to
taxi. Then when you add power for the run up you will position the
throttle farther open than normal to get the 1500 RPM that you need
since the carb is partially iced up. When you add the Carb heat it will
typically drop a bit and then back up to a higher RPM. The RPM Surge
you saw was simple the result of having more ice that you are used to

I see this quite often in the Champs I Fly and even Cessna/pipers
occasionally.

Brian
CFIIG/ASEL