Hi Daniel! At this time, the web dashboard doesn't support the brief stops feature which means that it displays all completed trips in their original (and unmerged) format. This can result in different MPG calculations for trips that include a brief stop. The dashboard also displays MPGs to the tenths decimal place while the iOS app does not (the Android app does however display tenths).
If you're seeing a major discrepancy beyond these two circumstances, let me know and I'll pass this along.