I've never done a leak down test so keep that in mind. TDC=Top Dead Center, what that means is that piston is all the way at the top of its stroke. Assuming that you have the engine turned TDC on the COMPPRESSION STROKE then both valves are closed. But as soon as you put compressed air into the cylinder the piston is going to move down (correct?) When that happens one or the other valve will get cracked open and thus an air leak.
IDK, Maybe I'm missing something or not.
If the lower pressure really bothers the OP then I'd do (or not) a dry then wet compression test and plan on a rebuild. Does it matter if it's a valve or rings? Personally I've never rebuilt a lower without also doing the heads and learned long ago on a gas engine, if it needs head work chances are it will also need block work.
