The relationship in play is:
IAS --> CAS --> EAS -->TAS -->GS
These are low level subsonic aeroplanes, so you can ignore the CAS-EAS corrections, but the rest is still there:
IAS --> CAS (PEC chart corrections)
CAS --> TAS (correction for density altitude)
TAS --> GS (corrected for wind).
You are describing two aeroplanes which I suspect don't have PEC charts available of any quality, nor will they necessary comply with the CS.23 and CS.VLA requirements for 5kn/5% accuracy of IAS; so any air data system on board will be already working with suspect data, allied to which that any internal ability within these cheap GA systems for internal calibration is largely useless.
So, you're working with poor data in the first place. Now add into that, the following potential handling related issues;
- For constant IAS (and thus CAS), TAS will reduce slightly in turbulence
- Unless you are flying absolutely level there will be a change in speed for any given power setting as the aircraft climbs or descends.
Overall, what you're describing sounds to me like the sort of small errors that you'd expect to see due to inaccuracies in handling and in airspeed measurement, and possibly changes in local turbulence levels.
But there is no relationship between TAS and head/tailwind component for a constant CAS value in perfectly level flight - just not possible.
G