I always thought the notion of not knowing what your co-workers make is silly and driven by major corporations to keep salaries down.
Is that is what is surprising?
Everybody gets paid different to do the same job because no one likes to share that information, it is faux pas to discuss. We all know what millionaires make, and what athletes make (public record) and it drives their market.
If I was in your shoes I would take the valuable information you received and use it to your advantage. If you feel you are doing more than "X" and now you know "X" gets paid more than you do, it will be easier to ask for that raise.
Corporate America has successfully managed to turn everyone into mindless drones.
You know why they make you sign that paper so YOU won't share your salary figure, they don't want people to compete for a higher salary.
Think about it, keepings salary info a secret benefits who?