The instructions for my unit gave a K factor and a note to say that calibration was required for most installations. Having used them in a professional environment in a previous life, the company had to calibrate each one individually (this was not in aviation).
Perhaps yours was a non-certified unit.
Shadin told me that most Floscan 201Bs they process come out around 29800 (IIRC that was the figure).
The one used in a G1000 aircraft would be a certified unit, with the K marked on it.
But as I wrote earlier these things are very sensitive to turbulence. The inlet pipe has to be straight (or have a very gradual curve at most) for the last 1-2ft, and it seems that a lot of installations ignore this. The result is a reading which is ~ 10% out, but the error varies.