When the throttle is nearly closed at idle or descent power, the air intake is more restricted than at fuller throttle settings. Air passing through the intake therefore suffers a greater temperature drop.
Also, downstream of the restriction caused by the throat and the butterfly valve the liquid fuel is introduced, where it will vaporise; when it vaporises it removes heat from the air (latent heat of vaporisation) and cools the air even more.
The temp drop can be greater than 30 degrees C - meaning warm, or even hot days of quite normal levels of relative humidity can be expected to produce carb icing at idle/descent power.
