His (TT's) write up on compression testing is in the tricks and tips section and yes he knows what he is talking about. Basically, the size of your motorcycle's combustion chamber is relatively small (compared to a car engine etc) and the volume of the hose, adapter etc can be large enough to significantly effect the reading you see at the gauge. Simply put, the volume of the adapter, hose and tester is added space for the compressed mix to push into, effectively lowering compression. Any tester will have some effect but ones with long hoses will yield particularly low results. Comparing the numbers produced with those from another type of gauge (like the gauge that Honda used to get its compression specs) is a crapshoot . . Other things such as, tester to cylinder head seal, battery power, starter motor condition, intake restriction (throttle must be wide open or better yet carbs removed) will effect your results. The most important thing to note in the test is the variation among cylinders. If you get consistent readings things are probably ok. If you get consistent but low readings, suspect your test procedure and or the increase in combustion chamber volume from the tester. . . .Also an engine that has sat for a while will likely yield poor compression test results. However, it is often the case that once the engine is returned to service for a few miles that ring and valve sealing will return to normal. . . .I would suggest getting the the bike going before tearing into the engine.
