Amim, it looks like others answered you privately, but I'll just put it on here for everyone.
550lbs is the rated BREAKING strength of the line - the amount of static weight that it will hold before breaking under ideal conditions. Safe Working Loads are generally considered to be 10% of Breaking strength, and that goes down a bit more if you're using block and tackle, or knots, or running it around corners, or whatever.
Real-world, dynamic situations are so much more stressful, and having loads drop because of line failure is so bad, that the 90% cushion is absolutely required. If you're doing things that involve suspending people, or suspending objects over people, or anything that relates to safety, it's not a bad idea to cut that in half again - that's one of the reasons that strength ratings on carabiners and other climing gear is so high - the idea is that even a maximum real-world dynamic load comes nowhere even CLOSE to the breaking strength of the gear involved.