That's because RK2606 and RK2608 aren't fully compatible.
RK2606 is for players whose screen is smaller than 2.4",
and RK2608 is for players whose screen is bigger than 2.8".
Also, RK2606's CPU is 100MHz, but RK2608's is 200MHz.
That is why some games like Snake and Head Space
can be launched both on RK2606 and RK2608,
but the most of them can't:
Tetris and Fighting Space can't be launched on RK2608;
Tanks and Pirates can't be launched on RK2606.
However, the Rockchip GDK is available for creating the new games & applications!
We can create the source code of the new game.
and then compile it for the several targets,
in that case there will be two versions of the every new game.