If you expand the version information on the planning board it shows you the number of issues with this version as the ONLY entered value for "FixVersion". If there are multiple versions assigned to an issue, this issue will not be taken into account for the displayed amount of issues.

Expectation: The number of issues for a version correlates to the search result for "fixVersion = x"

Update: The criteria does not seem to be issues with ONLY the selected version assigned, but something else. But what?