With the Lions' defence waiting for the linesman's flag, Adam Le Fondre tapped home the winner for the Royals fifteen minutes from time as the Championship's in-form team won their sixth game on the spin.
And while replays show that despite being initially offside, Le Fondre was indeed onside when he put the ball in the back of the net, McDermott conceded the rules are confusing.
"I still don't understand the rule to be honest," he told the club's official website.
"I think Robbo [Jason Roberts] was onside and Alfie was offside when it was played…and then Alfie goes in the box and is onside when he scores.
"I knew that was going to be a different kind of game to the ones we've had in the last two games against Burnley and Middlesbrough.
"I watched their game at Burnley, they were very good and made it difficult for Burnley – so to come here today, off the back of their result, it's a fantastic result for us.
"It might not have been the best-looking game to watch but, for me, it was a fantastic performance on a tough pitch to play on."