The documentation at:
https://www.mediawiki.org/wiki/Wikibase/DataModel#Quantities
says that:
"The exact interpretation of the uncertainty interval provided with lowerBound and upperBound is unspecified. Depending on context, it may represent hard limits on the value, or the interval may just describe the 66 or 95 percentile interval of a normal distribution."
This is problematic, as it depends on an implied context, rather than actually giving the uncertainty interval where is is available. Rather than relying on an implied context, it would be better to record the uncertainty level / significance level / number of standard deviations that the bounds are representing, where that is available. Most scientific publications will include a mention of this, so this should be widely available, and it should be possible to represent this in Wikidata's entries.