It was cold so not all the fuel was getting burned, causing the low idle and probably rather a lot of white smoke out the tailpipe. Once you got the combustion chambers a little warmer, the combustion efficiency went up, releasing more heat and doing more work so you're idle went up.

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.