Differences in the desktop and the service version of Power BI can occur because of the following reasons:
Resolution and Screen Size: The alignment of different display resolutions visualizes effects on it.
Browser Rendering: How visuals appear in a browser may differ from how they are shown in the Power BI Service.
Performance Settings: Cached data or settings may not be used by Power BI Service as a Desktop.
Data Refresh: Both Desktop and Service should be checked to ensure the same data refresh settings.
Page Size and Layout: Double-check layout settings in both versions.
Test both platforms with the same data and settings to ascertain consistency.