My performance against an IM OTB in a simultaneous

I got in a simultaneous against an IM (btw, this IM hadnt played for half a year), and I lasted 16 moves until I asked for a draw because it was near midnight and I had to go to school the next day. She played Queens gambit, and I only knew the first 4 moves of theory until I accidentally deviated, and I played the rest of the theoretical moves based on calculations and some strategic analysis. According to stockfish, she kept an advantage between +0.2 and +0.5 in some moves, and I only lost a pawn in the entire match (I blundered a second pawn, but she didnt take it). After a while, I looked around me and I realized that she had already beaten everyone but 7 people who had very high FIDE and ELO ratings, and me. For reference, there were over 50 people initially. I have a FIDE rating of about 650. I know that this sounds like I'm bragging, but I just wanna know how it's possible that I was the only lower rated person left after playing for over 3 hours against an IM, and how did I manage to do that. It's pretty confusing. Sorry if I wrote too much. 

thanks