In a tournament players who sign up with intention to win usually take it serious, meaning they practice the matches and do whatever they have at hand to win including more effort on the game that you play officialy. On a showmatch you may play rusty, unprepared, or without interest as many players currently would do.
It's funny in Warband there was some similar syndrome, people would claim to be better than other teams or players because of scrim results or ft7/ft10s(if it was melees and not just 1v1). By the end of the day whoever won the tournament truly showed to be better. Fact is that FT was beating AE non stop(except some days where they stroke back) during WNL on scrims, even with some stomps, yet on finals they did beat us and proved to be better than us.
It's applayable to any scenario, think about football itself, who gives importance to 'friendly matches' or even non determinant official ones, it doesn't show anything compared to winning determinant league matches or knockout in any cup-type one. Just as simple as that to get the difference
In my opinion it should not be hard to solve, it's the mechanics that are mainly unbalanced. 80% of cav mechanics are not properly done, Ranged accuracy factors and kiting is not properly done, and infantry feints/block anim blend time combined with armor absorbtion/damage calculation are awful right now. There's also some general issues that apply to all 3 classes. But once those are fixed it wouldn't be hard to apply some minor issues for all 3 classes & real balance in terms of gold and equipment to each class.
Doesn't matter anyway, we kept spamming this all along and Taleworlds will take time to fix it if they even do it, I guess the least we can expect sometime soon is some experimental tournament by them, in which there won't be any "preset rules".