I have read these lines in a book 

*Work done by total forces(internal or external) is equal to change in kinetic energy of the system with respect to centre of mass.*

Basically $W\_1=\Delta KE$\_{w.r.t CM}

Also i know 

$KE\_{total}$ ( from any frame of reference)$=KE\_{w.r.t. CM}+KE\_{CM}$.

Now what if we observe the system from another external reference frame, from whose POV centre of mass has some velocity and here we apply an external force which changes the velocity of centre of mass(thus ke of centre of mass) and also changes the ke with respect to centre of mass of system then this entire change according to me is equal to the work done by this force.
$W\_2=ΔKE\_{cm}+ΔKE\_{system wrt cm}$

But $ΔKE\_{system wrt cm}$ depends on relative velocity of particle(which is independent of F.O.R.) hence in both W1 and W2 this factor is same so clearly W1 is not equal to W2, so why on changing frame of references we obtain 2 different amount of work done?
I know about the fact that work done is relative but, how exactly is it relative in this case.