It has long been a weirdness of this game that the player is so widely unpopular at the start of the game. I normally don’t read the text at game start but for whatever reason I did this time. 10% is not a majority.
I suppose the wording could refer to me winning a majority government with a margin of support 10% greater than that of the official opposition, however, given the greyed out popularity bar at the top left I don’t think that’s what the game is referring to here.