There are actually two distinct performance engines calculating performance results for a flight at the same time - ForeFlight’s cloud-based engine that requires you to be online in order to access it (the online engine), and ForeFlight’s built-in engine that can be used when disconnected from the internet (the mobile engine). Due to various factors, these two engines will return slightly different performance results for the same route.
The performance summaries on the Maps and Flights views and all the performance numbers in the body of the Flights view (in the Fuel and Weight Verification sections) are calculated by the mobile engine. The performance results in the NavLog and Route/Altitude Advisor are calculated by the online engine.
Of note, the Navlog and Route/Altitude Advisor and associated performance calculations are not accessible when offline (aside from cached routes and wind data), while the other parts of the app will continue to calculate performance when offline.