Observation: there is always a portion of tryptophan that is devoted to vitamin B3 synthesis.
Interpretation 1: we are chronically short of vitamin B3, and some tryptophan, although essential for protein synthesis, and for its downstream products, including neurotransmitters, is being sacrificed to meet this more important need.
Interpretation 2: need for yet another product made off the vitamin B3 synthesis pathway is driving the degradation of tryptophan and the synthesis of some more B3.
Analogy: running the mevalonate pathway and making some more cholesterol when cholesterol is already in excess – look to any of the other products being made off of this pathway that may in fact be deficient. Supply those nutrients with supplementation and total body pools of cholesterol may just decrease.