Analog data is read, demodulated/etc, played back. It doesn't match each time.
Capture cards often don't ingest the same either.
If you want to get even more anal, notice how the IRE on both recorders sucks. The only DVD recorder that is accurate is a certain RCA, but it has other limitations preventing it from being "the best" recorder (namely no cNR, making it awful for VHS).
You're about where I was back in 2002-2004, when these devices were all new. I quickly saw that quality and accuracy was all over the place. That's because no accuracy exists. It's analog. Analog isn't accurate, especially when mixed with digital tech. You do the best possible, but also understanding where the brick walls are.
Even digital isn't accurate, but merely far more accurate.