Seed has absolutely nothing to do with the relative strength of teams. I find it problematic to make any verdicts of win probabilities based on seeds.
Really? Absolutely nothing? I agree that seed might not be a perfect indicator, but the team that won more games over the span of a season will have the better team in the vast majority of the cases.
Probably. Maybe. But you won't even know how much better (hence the stress on 'relative'). The fourth seed can be almost as good as the top team (and will be, in the long run), but in some leagues it can even be a bot.
Plus, it is always possible that a stronger team lost some games because they were concentrating on the tournament.
In view of all this, I find it extremely problematic to commit to any sort of percentage probability of how often the lower seed should upset the top seed.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."