What is the difference between planned true airspeed and actual true airspeed? My electronic E6B does calculations for both planned and actual true airspeed (yielding different results). I was told the difference has to do with assumptions about temperature, but the manual doesn't give any details. What is the difference between the two?
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.