Ah I see. I assumed you had the common sense to add to Xbox any game that got good reviews on the PC, PS4, or Switch side. In that case Xbox isn't as bad off as this graph shows.
That would be sisyphus work to check every game on every system. I only mirror the metascore data. Without judgment, which scores should count and which shouldn't.
What do you mean by tie ratio?
Tie ratio is the number of games bought per console/system on average:
The VGC tie ratio doesn't include digital sales, so it should be a bit higher. Nevertheless most gamers will only buy ~20 games for a system on average. So it doesn't matter THAT much if 300 or 500 good games are available for the system.
Also a few questions about your graph...
1. Do game expansions count as an individual game? For example if WoW gets a new expansion does that count as +1 game for PC? Or if Sword/Shield gets new DLC does that count as +1 game for Switch?
If the expansions are popular enough to get enough good reviews, they are counted. I only mirror the metascore data. Without judgment, which scores should count and which shouldn't. But since all platforms have a few games with good reviewed expansions, that should even out.
2. What's the ratio of 2D Indie games to fully fledged 3D games?
I don't care. A good game is a good game, no matter if it is a 2D indie game, a 2.5D indie game, a 3D indie game, a 2D AA game, a 2.5D AA game, a 3D AA game, a 2D AAA game, a 2.5D AAA game or a 3D AAA game. Also the transitions are fluid.
3. Do game compilations count as their own games? For example: Do all the games in Super Mario 3D All Stars count as one game for Switch or as three games? If a game gets released in both a compilation, and in a standalone release, do both the compilation and standalone release count as a game? Would Control and Control Ultimate Edition each count as +1 release for their respective platforms?
If the compilations are popular enough to get enough good reviews, they are counted (as one game, since the compilation gets the score, not the included games). I only mirror the metascore data. Without judgment, which scores should count and which shouldn't. But since all platforms have a few compilations and most compilations are released on all popular platforms (f. e. the Borderlands Handsome Collection), that should even out.