It is simpler than you think. What does assumed temperature or flex do? It actually reduces engine thrust and take the runway length as an advantage to get airborne. I am not familiar with the Boeing performance software. But if you use Airbus flysmart to calculate takeoff performance, it gives you two sets of data. The data for a TOGA takeoff and the data for max flex or your preferred flex.
One of the main differences between these two data is the remaining runway length after an accelerate stop (rejected takeoff). When TOGA is used, the margin is always higher than flex because the former gives you a better acceleration and you get airborne a lot quicker.
So, to answer your question. A contaminated runway has less braking efficiency. Thus, you need more runway length to stop the aircraft. A TOGA takeoff gives better acceleration and thus gives you more runway to reject a takeoff.
As others have mentioned you can do a derate takeoff. Here, the thrust of the engine is reduced to reduce Vmcg/a (minimum control speed). A reduced Vmcg allows you to reduce V1. And a reduction in V1 means you have more runway to stop the aircraft. Hope this helps.