The main use case for the Nearby tab is for walking around. The app will update when you move a short distance to try and show you the closest article to you, which is important for places like cities with many landmarks and buildings right next to one another.
However, you might be using the app when riding on a train or bus, in which the quick updating would become impossible to use. By the time you decided to click on an article, it would have moved down or off the list. To compensate for this, we use live data from Apple's motion coprocessors to see if you're in a moving vehicle. This improves the accuracy of knowing when you're moving fast when combined with GPS data.
When we determine you're moving at a fast rate, we switch into a manual reload mode, where you can force a refresh of your location data. Otherwise the results shown are of your last location before we detected you moving at a fast rate.