I don't know anything about a HT US sensor, and the referenced link shows something for the RCX, I can't see that it is for the Lego NXT USS.
The NXT USS has a upper limit up to 255, each unit corresponds to 1cm.
The first 6~8cm are too short to be reliably detected, but then e.g. the range to 254 cm distant walls is trustworthy.
255 stands for "255 up to infinity", and often values of distances below 6cm also show 255 (that can be eliminated by a minimum filter, while values equal to 255 should be dropped for evaluation).
NXC CHESS for NXT: http://www.mindstormsforum.de/viewtopic.php?f=70&t=6790
indispensable for NXC + EV3-C: easy + trouble-free network & sensor+motor remote control for rs485, BT, USB!