After some digging, I think it’s an artifact of how the elo skill rating system works. When you’re at the pinnacle of the game, you’re better than most players, and the game knows it. It tries to give you the hardest opponents it can find, but in reality there are few players who can give you a good run for your money. This means top players are winning most of their matches – but they’re also not exactly the hardest matches (not equal strength on both sides) so winning gives less (you were expected to win) and losing hurts more (you weren’t expected to lose). It’s a bit like odds in a horse race – the strongest horses are most likely to win, but don’t have amazing payouts because when they win it isn’t a surprise.
Though that’s probably why this occurs, it doesn’t mean it feels good and we’re definitely not happy about that. The ideal solution would be to only offer tough matchups for the strongest players, but sometimes it is hard to find such a matchup because the number of players at that level are relatively few (like a pyramid). Boosting cards helps some at lower levels, but at the highest levels of the game boosting is harder as there are fewer opportunities to boost to create a compelling, even-keeled match.
I don’t think there’s an obvious (and fair) change to improve this yet.