Incorrect data in GA4 realtime reporting API vs UI

Question

When I use the Google Analytics RealTime API, I get a difference in the data between the API response and the data from the GA4 UI dashboard.

I want to retrieve the active users for the last 30 minutes. By default, this date range is used on RealtimeReport.

$request = new AnalyticsData\RunRealtimeReportRequest([
    'dimensions' => [
        ['name' => 'deviceCategory'],
    ],
    'metrics' => [
        ['name' => 'activeUsers'],
    ],
]);

But almost every time I get a difference in data between the UI and the API.

API: 936 users / UI: 1290 users

I've tried using other dimensions (country) or removing the dimensions to keep only the metrics, but it doesn't change anything.

Do you know where this difference in data comes from?