The "Even teams" or "favored team" indicators is not based on leagues, but on your true match-making rating.
Leagues are really just a display convenience for which percentile "bracket" that battle.net thinks your skill level falls into. However skill levels change and battle.net is not always right on its first estimations, so naturally leagues change. But they don't change instantly; the system looks for consistent trends before promotion/demotion.
Your hidden match-making rating, however, is battle.net's true estimation of your current skill. This is what's used to match players for games, and this is where the "even teams" notice comes from. It updates after every match. It is unfortunately hidden to prevent players trying to cheat the system.
In your specific case, there are many possible explanations. For example, your opponent may have placed poorly initially, but then had a winning streak, so their match-making rating was 2 leagues higher than their league. Perhaps their true skill level is somewhere in between, and the system will gradually determine this.