33.3 KHz rather than 37.5 KHz
It is well known that a transducer changes the resonance frequency under pressure. Without knowing the ULBs construction or other reasons, I can imagine that the design goal is to get the highest signal rather than keeping the frequency on 37.5Khz. The ULB may be pretty simple (kiss) and only kick the transducer at right timing and keep it running for a predefined time at the current transducer resonance. The frequency is not as important than getting the best signal strength.