Calculating Headwind/Tailwind component

Most flight computers nowadays calculate wind vector, but only few calculate headwind/tailwind component, which, in my opinion, is almost equally important, since it tells you if your glide over terrain/final glide is going to suffer or benefit from the wind component, as well as help you estimate your task speed. When the vector wind is cross, it is hard to estimate if it results in HW or TW component (as it is factor of wind speed, direction and glider speed).
Thankfully, some flight computers such as 302 provide TAS, so my solution was always to subtract TAS from ground speed to determine HW/TW component, but I am surprised that not all flight computers use this simple calculation to provide instantaneous HW/TW component. After discussing it with some, it was suggested that my assumption is wrong, and that HW != TAS-GS when cross wind is presented. I am not sure why though.
Thoughts?

Thanks,

Ramy