What is the difference between planned true airspeed and actual true airspeed?
It's very possible that your flight computer's TAS calculation is expecting calibrated airspeed rather than indicated airspeed; the difference between these two can often be enough to affect your results by a meaningful amount.

Most aircraft do not have a formula for the conversion, or enough information to convert airspeeds other than those specified in the manual. This means that the "CAS" used for TAS calculations is often actually IAS.