Calculating the voltage generated by an 40KHZ Ultrasonic Tranceiver
I don't know for sure, but one possibility might be that the quoted SPL measurement has a condition attached, such as a measurement distance. It ought to, and a common distance for SPL measurements is 1 metre.

In which case the voltage measured at only 125mm distance could be expected to be greater.

If acoustic power varies as the inverse square of distance, then voltage will vary as the inverse of distance, and this would lead to 8x the expected voltage at 1/8 distance.