The 550 pound strength is the ultimate strength of the line under ideal conditions in pure tension.
Knots and splices greatly reduce the effective strength of a line significantly. A simple bowline reduces the strength of the line by 40% bringing the 550# strength down to 330#
Other factors reduce line strength as well. Shock loading the line may not break it outright, but it can damage the fibers and reduce the strength. Age is probably the worst thing - everything from mold, mildew, wet and dry rot will take its toll on the line and UV degradation is pretty awful, too.