I wouldn't think it would be cable creep. Nor the string for that matter. They just don't creep that much.
Were you using the same chrono? How was the lighting? Lighting can have a profound effect on what the chrono sees crossing over it. Who knows? Maybe the chrono is wrong both times. Too fast the first time and too slow the second.
I don't know about you, but I can personally tell a difference in that much speed. 318 is buzzing along pretty well and you should be able to tell a drop of 56fps.
If you had the bow set up and sighted in then you should be able to just shoot it and see if your sight marks or pins are showing the speed loss. If the pins/sight marks on on then I wouldn't worry about it.
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