How long does it take for custom KPIs to update? I’m seeing a discrepancy between when I click “validate” and what my dashboard is showing.

When I click the ‘validate’ button after creating my KPI using the JSON editor, it says the KPI value is “264”. Then I click “save” and go back to the list of KPIs and it says the value is “256”. Do these KPIs get calculated once per day and I should just assume it’s a matter of time before it gets updated, or is there something else going on here?