Better battery life by factory reset?
You cleared Android's batterystats file by doing a factory reset, specifically the file /data/system/batterystats.bin was deleted and regenerated. This file holds a calibration for your battery. Before you did a factory reset, your batterystats was probably miscalibrated. It's a common practice, for rooted devices, to delete this file every now and then with a fully charged battery. This correctly recalibrates batterystats.