I’m pretty sure Salty explained it somewhere but it wasn’t front and center. If it wasn’t Salty it was someone else who represents the devs. I forget the exact details but it was something like this:
If all you do is Casual PvP and never ranked, you don’t get any of the revenge/whatever battles with increased rewards. But gaining PvP rank tells matchmaking to give you stronger opponents, which also affects Casual matchmaking because of course it does.
So smart people would fight 1 Ranked PvP battle to win so they’d get in the Revenge pool, then lose a few battles to look “weak” to the matchmaking system. Then they’d fight non-stop casual PvP, using cheap refreshes to seek either very easy or Revenge battles to get more gold. (I didn’t even think Revenge battles worked in Casual? Maybe they did it because losses are negative points. I don’t know. It’s all messed up.)
Anyway, the obvious problem is your actions in Ranked could affect Casual, so the obvious solution is to make all PvP fights pay out worse rewards. That way nobody’s happy.