Meaning of uncertainty value returned by Soilgrid API

I am trying to wrap my head around the uncertainty value and quantiles returned by the Soilgrid 2.0 API. I went through this document, but cannot find a clear answer.

As an example, I issue the following request to find pH of a certain (lat,lon) value:

http://rest.isric.org/soilgrids/v2.0/properties/query?lon=1.587148&lat=43.371551&property=phh2o&depth=0-5cm&value=Q0.5&value=Q0.05&value=Q0.95&value=mean&value=uncertainty

and get as results:

  'values': {'Q0.05': 52,
   'Q0.5': 77,
   'Q0.95': 84,
   'mean': 73,
   'uncertainty': 4}

(deleted the rest of the response since I don’t see anything else relevant).
According to the FAQ, the interval [52, 84] contains the true pH value of the soil with 90% probability.
I’m interested in cutoffs at 5.5, 6.5 and 7.5 of the pH values, so this interval is not very informative. I sampled over 2000 points and see that this is the typical case, not an outlier, i.e. these intervals are always very wide.

Am I interpreting this correctly?

On the other hand I see the uncertainty = 4 value included in the response, and had hoped this would somehow offer an alternative, but I can’t find what the exact definition of it is.

How is it calculated?

How can I use it?