It is because most of us only determine the strength of the team based on their squad quality. If many popular players in the team, we will say it is a stronger team. However, in the match, sometimes there are other factors that can bring an influence. For example: the teamwork, the mentality of the team, and the tactics. These factors are not easy to determine, that's why our prediction is possible to be wrong although we choose a stronger team.
Exactly, and that's why I said that in this competition it is difficult to determine who will qualify for the final round and take home the UEL trophy. Because other things can change the match in a short time. Like the experience of Atalanta who initially never thought of getting the UEL title and making Leverkusen proud, but in fact in the final round Leverkusen did not have a great mentality and made Atalanta win the title after having a very strong mentality of players.
In the match Real Sociedad vs Man United, Man United surely has more popular players. But Real Sociedad sometimes can surprise us because the team from La Liga isn't easy to defeat. I think Real Sociedad can defeat Man United if they are not ready to deal with Real Sociedad tactics.
Real Sociedad can be very fierce when playing at home. Meanwhile, Man United again has problems with several injured players and will most likely be absent from the match. Most likely in the first leg Real Sociedad can get a win and Man United must reply in the second leg when playing at home.
BTW, I see from H2H, MU once beat Real Sociedad when Ronaldo was still with MU and was only able to score 1 goal. I am more convinced that MU must admit defeat in the first leg later.