iPhone storage increasing over time
There are a few potential reasons for this.


If you installed a system update, the update data will be removed post update which can result in a large amount of space being.
Apple have begun to implement several space saving features into iOS, which offload unneeded content to iCloud Drive or just remove it.
You may have removed some optional system content, such as dictionaries, which freed up space.
Rebooting your iOS device can sometimes free up some space, as iOS is based off of BSD so logs and temporary data may be expunged on reboot.