As you drop fuel in cylinder there is less thermal energy. At some point it means that its not generating enough heat to do anything. This is the only thing that saves a motor that is WAY LEAN vs one that is sort of lean.

Mega rich- EGT drop
Mega lean- EGT drop

Sorta lean (13-13.5)- EGT increase.

 

that is crazy. I've yet to max out the injectors for my tunes. Same setup.