My 13,500 raid wins (longest win streak: 65) say your 11,500 raid wins are spouting nonsense. If you find that the board dictates the raid outcome for you, then you’re running too colour heavy, making you vulnerable to disadvantageous boards.
…and there we have our explanation. If you want to rely on what the boards give you for your wins and losses, stack your colours. If you want to be able to win with any board, run rainbow. You sacrifice brute power but gain consistency and maximise the chance to use a bit of skill.
Which, I think, was what you were hoping for, wasn’t it? You’re welcome.
(For what it’s worth, these days I often stack two deep against the tank. I do it because it’s fun and I’m lazy, but I recognise that this approach will result in some bad board losses, and that some of my wins will be gifted to me.)