I read the list of PlayStation games not compatible with PlayStation 2 and I noticed that Tomba! was on that list. But I played Tomba! on my PS2. So, after a bit of research I discovered that I was able to play it because I have a Slim SCPH-77001, with PlayStation driver 2.0, which along with the other slim PS2's uses a different way to achieve backwards compatibility with original PlayStation. But I was puzzled at why out of so many PlayStation games there were a certain few that did not run on the original PlayStation 2. As well as some others that did not work on the original slim PS2.
I want to get a detailed explanation on how exactly the different models of the PlayStation 2 achieve backwards compatibility, and why exactly does a game like Tomba! fail to execute on that particular PS2? Also, there are apparently some PS2 games that don't work on some other versions of the PS2, an explanation on this phenomenon would also be very much appreciated.