Reference valve and seat installation, are you suggesting that it is the overhaul shops not using the correct technique or the OEM?
I think both. I think once in every 10 or 20 cylinders "something happens" to make one out of limits in one way or another. Perhaps a compounded error, where both parts are within tolerances, but when assembled together they're not. I don't honestly know, but we didn't see much of this on cylinders manufactured before about 1991. CMI (TCM then) had a strike, where managers went down on the floor, and it was also about then that some machinery upgrades were done. Some very old machinery that had been doing the work right might have gotten replaced by "modern" machinery that wasn't so good. All supposition, but the results are clear. Efforts to blame LOP are nonsense.
CASA has given advice to the industry through ADs and AWBs about boroscope evaluation etc to assess cylinder condition when compression is below the 60/80 differential.
Depending on how that's implemented, it's probably a good thing! Borescopes have been around a long time, but it's only in the last few years they've been cheap enough for even owners to have. Under $200, with digital picture-taking ability direct to a computer. If a shop or a mechanic doesn't have one, I'd look for another mechanic.
I have seen comp as low as 20/80 improved to an acceptable level by running the engine and rechecking.
Low compression from blow by can be simply a matter of the ring gaps aligning, well that was one theory.
YES! One of our slides shows compressions all normal (rough engine). A second compression check was done after flipping the prop through few times, and that test was 0/80. Sticky valve, of course, sticking on some strokes, but not others. A single compression check should NEVER be taken at face value. Always borescope it, if that's good, run or fly the engine, or fly it hard, and check it again. CMI did good, with the latest SB. It's now obvious that we've all pulled far too many cylinders based on compression alone for the last 100 years!
John Deakin