Export the clip out without a grade and look at it in FCPX. Is it actually different. Brent is right. If you are using the computer display and comparing the way two NLEs approximate the image, you are going to think there is a problem.
Resolve is designed to pass the correct signal via a Decklink card to a calibrated external monitor. Using the computer monitor to make a judgement is always likely to yield variable results. And who knows which one is closest to correct – FCPX or Resolve. Probably neither