Tim and the runScribe team are the experts on this. However, I can tell you as a frequent user and having communicated with support, that they are still optimizing the battery model.
Operations like running, which use the sensors and write to memory, syncing, and to a lesser degree making BT connections draw down the battery.
Checking the battery immediately after running or syncing might show an artificially high or low result.
If a set of batteries have less than 15 hours on them, they should be good for more runs even if the app indicates they are low.