Cambridge 302 Indicated Airspeed

I fly a Ventus CT with a Cambridge 302 and an Ipaq using Winpilot.
Flying yesterday I checked the IAS shown on the 302, I noticed that it is
giving
totally different readings to the mechanical ASI.
eg at 50kts on the mechanical, the 302 showed 40kts
at 70kts the 302 showed 63 kts.
I did not check at higher speeds to see if the difference reduced further.
I am reasonably sure the mechanical ASI is probably correct, (if it was
10kts out on finals
it would be an interesting approach) !!.
I checked the IAS as I have been doubting the wind shown on Winpilot, if it
is using the IAS fron the 302
and that is in error, then that would explain the wind problem.

Does anyone have any suggestions as to what can cause such a difference
betwen the mechanical
ASI and the 302??
Thanks
Ross