With the notation, it'll be underlined if your cursor is on that choice.
Where it shows the # of total games, it won't let you click it if the number is too high. There's a cutoff point. I think it's like 10,000? Looks like that's what it is.
You can think of it like, aside from being more games than you'll ever want to look though, the line hasn't diverged far enough yet for the games you'd look at to be of any use. You wouldn't come away knowing more than you would if you just took the most common book move at face value and clicked instead on a game further down the line of play.
I have searched Help for this question but can't find anything. In Game Explorer sometimes one of the potential moves is underlined. Also for the number of games with each move the most common choices aren't underlined but those further down are underlined. I can't figure out the reason. Thanks