The 738 is an aircraft designed for automatics. It has STS which basically is a speed stability
augmentation system designed to improve flight characteristics during during operations with low GW, aft CG and high thrust when a/p is not engaged. STS monitors the stab input and thrust lever positions, air speed and vertical speed and trims the stab with the a/p stab trim. The STS operates counter to pilot inputs.
Somewhere therein may lay the answer to this puzzle.