When it comes to tuning and timing not all of our tools are exactly the same or give us the same results. I saw a timing light tests at the races one day were three different timing lights gave two different results
One of the main things I've learn is to use my tools on my cars and engine and look at the time slip and spark plugs to get the best results regardless of wat the readings look like
I had eye opener many years ago when a good division 7 SS racer told me his 415 HP M.W. motor in his 1963 Belvedere like 42 degree total timing revved above 3500 RPM, my stocker with the same combination didn't like any thing above 36 BTDC or less than 34 BTDC. I found out later he didn't build his engines, his sponsor did and they had change the early timing cover to a new later year one, the early ones have the zero mark at the top of the timing tab and the later ones, after maybe 1965 or 1966 have them in the middle of the tab, which reads incorrectly with the wrong balancer. I always verify TDC on the dampener and timing covers compare to TDC off of the #1 or #6 piston at TDC before putting the heads on