-Idle changes upon reaching operating temperature.
-Smooth idle under both temperature related conditions.
I'd say JW51 nailed it with the heat riser doing its job.
Proper setting of idle speed requires a fully warmed up engine.
In your case, adjusting the idle speed after fully warmed up will result in a slightly higher idle on a cold engine, which is desirable for speeding up the warm-up process.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission