Say your performance-limited weight on r/w 28L is 100T at 30C.
But at 20C allowable TOGW will probably be higher, ASSUMING you're not at a structural (airplane or runway) limit. This is because to reach V1, Vr etc. you don't have to accelerate to the same TAS or GS. (The wing is flying in cooler air).
Now if your actual TOGW is 90T at 20C, and you flex to an assumed temp of 38C (I'm guessing here), you'll be flying a profile equivalent to flat rated thrust at 30C and 100T.
Both the reduced GW and the lower OAT contribute to the thrust reduction from the 30C/100T condition.