Hi,

Personally, I think that the engine will be turning over to slow, during the test, to push enough air into that big of pipe and get any sort high of back pressure to cause high readings.
If it where the case it would have to be capped!
High readings could be the camshaft but only on the exhaust valves since it is getting air in.
But why only the exhaust valves is my question then?
Cam “Lift” on both are about the same distance and the exhaust valves are bigger to allow for the expanded gases to flow out.

A plugged exhaust pipe will cause lack of power especially during acceleration.

What are the symptoms that you are having to warrant the compression test to begin with?

Phil