varying RPMs, throttle cable issue?

[Merkycury]

I have a 2003 50HP Mercury bigfoot. I took it out for the first time and it was running at 6080 RPM @ WOT. Then the next day it was running at 5500 RPMs, again at WOT. Then after slowing down and going back to WOT it went to around 5800. Would this be a cable issue?