I doubt that things have changed much, but a standard length for string on traditional gear has always been 3" less than the length of the bow. In other words, of your bow is 60" the string should be 57" if you're going with a standard setup.
My best guess from what you've posted is that the replacement string you bought was either put in the wrong packaging or the packaging is mismarked. If you're measuring 55.5 it's plainly too short.
Some people like to play around with different brace heights so may change the string and go 1/2" longer or shorter, depending on what they're trying to do, but 57" is the suggested length you need. However, note that the packaging should be marked 60" AMO. When you return that one or just buy another one I would suggest you have it measured before you leave the store.
If I may be sold bold, may I ask why you are replacing the factory string?
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