Not sure I agree. Here's why. ALL you're doing is tightening down some fasteners and it makes a difference. You have all the rest tight, but when you loosen those two, that's when you get good compression. Also, when they are tight, you STILL have compression, just a very small amount. So that means something's touching "just barely enough" to affect the compression on that cylinder. Also, when you swap out the steel valve cover, it's not as tall so it affects it even more. You're touching "somewhere", there's just no evidence. Have you inspected the adjuster nuts REALLY GOOD? Those would be really difficult to see witness marks since they are a dark color. Maybe even try some modeling clay on the rockers on the tops of both ends.