Having looked at the photograph of the offending wooden post and the advice contained on Page 118 of the Owner`s Handbook, warning of the limits of ultrasound measurement, including a thin or wedge shaped objects, the following thought may be relevant.
Could it be that almost all of the Ultrasound beam was reflected away from and not received back by the relevant sensor, due to the shape of the post. That would mean that on its own, the thin leading edge of the post was borderline in respect of the signal strength received back by the sensor.
I am not sure how Ultrasound works compared with Radar but assuming similarites, military Stealth Aircraft and Ships produce a very small radar return, partly based on the the special angled shape of the bodywork. This may or may not help to support the above theory.
Standing by to be shot down by experts!!