The number in a motor designation is the average thrust not the maximum.

In most cases a motor with 5 newton seconds of average thrust would have a lower maximum lift-off weight than a motor of 6 newton seconds but, the C5-3 is an exception. It has a cored design that gives it a very high initial "spike" then drops off to a lower , and slower burn which gives it a lower average thrust than the C6-3. If you go to http://www.thrustcurve.org/ you can compare the thrust curves of many different motors

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Jeffrey Deem
NAR16741
CIA section 527