Capped means - ready.
No IFs or BUTs.
It is ready (meaning - this particular honey is ready).
Refractometer is only useful when you want to extract uncapped honey (because you can not ask the bees the same questions in human words).
Refractometer just gives you the #.
You then interpret the # using some average, good practice guideline to decide - ready or not (it is a sledge-hammer approach, but usually works).
Us humans are not capable to decipher exact properties of the exact honey we are looking at (the ready # will be different per the particular honey).
The determining factor of whether honey is ready to extract is moisture content. Honey above 18.6% moisture is in danger of fermenting because of the natural yeasts in
honey. The higher the moisture above 18.6, the sooner it may ferment. While it's a good rule of thumb to assume honey is ready to extract if capped or mostly capped, it's not
a guarantee of moisture being in an acceptable range. It was a big surprise for me when I purchased a refractometer years ago and discovered it wasn't unusual to sometimes have capped honey test high
in moisture and uncapped honey test low. Beeswax cappings are permeable and allow moist air to pass both ways. That's why it's possible to pull moisture from capped honey using a dehumidifier.
While it's true that viscosity of honey may vary some depending on floral source, the 18.6 threshold still applies. I prefer that mine is somewhere between 17.0 & 18.0 just to be extra cautious.
Your honey may be fine, but I would suspect high moisture if it seems really thin and runny. Most local bee clubs have someone with a refractometer willing to test honey for moisture content.