The temperature in the venturi and at the throttle plate when it's closed can be more than 70°F lower than ambient, which explains why carb ice is possible when the OAT is 100°F. Air accelerates through the venturi and past the throttle plate's edge when it's closed or nearly so, and higher speed means lower pressure, and lower pressure means lower temperature. Add the evaporation of the fuel being sprayed into the airstream, and more heat is absorbed from the air, lowering its temperature further.
Certfied airplanes have to meet this requirement:
Sec. 23.1093
Induction system icing protection.
(a) Reciprocating engines. Each reciprocating engine air induction system must have means to prevent and eliminate icing. Unless this is done by other means, it must be shown that, in air free of visible moisture at a temperature of 30° F.--
(1) Each airplane with sea level engines using conventional venturi carburetors has a preheater that can provide a heat rise of 90° F. with the engines at 75 percent of maximum continuous power;
(2) Each airplane with altitude engines using conventional venturi carburetors has a preheater that can provide a heat rise of 120° F. with the engines at 75 percent of maximum continuous power;
A system that can provide a temperature rise of 90°F is a serious source of heat. A lot of homebuilts can't come anywhere close to that, with the result that engine power loss due to carb ice is far too common. Another factor there is the number of pilots who think carb ice is only a wintertime thing.