There are several things to consider here:
First, the formula for FX-37 has been published with errors more than once, the most prominent being tenfold to much benzotriazol which would well account for a loss of speed.
Second, you don't know if everyone reporting on filmspeed measured it or if this was based on guesswork.
Third, how did you measure the effective speed?
Fourth, do not expect a speed increase as high as adverstised for several FX-formulas, I've yet to see it.

best

Stefan