The valves seal the combustion chamber.
The stem seals are above this in the valve guides in the head so can not affect a compression test. Oil is pumped to the head to lube the cam and return to the sump through oil ways. Worn stem seals let it seap to the back of the valve and enter the combustion chamber when inlet valves open or is pushed out of exhust when exhust open.
Hope this helps
Stephen