How is iPhone Storage automatically cleared up?

I'm using an iPhone 6s with iOS 10. I have a few apps, but most of my space is taken up by photos and videos. Now I do run low on memory, sometimes I have just 500MB left, but I just let it be. If I get too low, about 100MB, I free up some space (upto 1GB).

I've noticed this a few times now: Sometimes I get a low storage warning, and I see that I have around 100MB left, but I wait till I have time to free up some storage. But after few hours, when I get time to free up, I see that my phone now has cleared up some storage on its own. Means I now have around 1GB free (for example).

My question is how was the storage cleared? What was deleted "automatically" by the phone to create space?

UPDATE: When I said my phone cleared up space from 100MB to 1GB, that was an example of sort, it usually clears up around 300MB on its own. grgarside's answer(regarding clearing of cache) is the most applicable to my case and hence I'm marking it as accepted.

App containers include a folder called Caches. This folder can be emptied by iOS when storage becomes low.

iOS chooses apps which you use rarely and apps which have large quantities of cache stored to be cleaned first. iOS shows the app name as "Cleaning…" briefly on the home screen when this process takes place.

There are other system caches which can be cleared too, but caches within containers are usually the largest and first to go.

Is your iCloud Photo Library on? If so, then iCloud could be backing up your photos for you and removing the copies of the older photos that are backed up, from the phone.

You can check that by clicking on Settings -> Your Name -> iCloud -> Photos and make sure that it is on. Click on Photos and check and see if Optimize iPhone photo storage is checked as well.

While application Caches are indeed part of the cleanup process, an increase in free space this large is typically the OS deleting a pre-downloaded newer iOS update. In your case, this would be iOS 11 (which is currently showing at 1.24 GB).

You can also manually remove an iOS update that has been pre-downloaded from within the Settings > General > Storage page if you need some quick space for taking photos or videos.

Yes both Photos and Apple Music have an Optimize feature. In IOS 11 there is even a setting to offload is unused Apps.

Personally i have also realised that everytime my phone goes low on memory, and it clears on its own, some functionalities like built-in dictionary will not work when i do look-up on words. probably because the system temporarily turns it off or deletes it to clear space. At once i had redownload the dictionaries but most times they start working well when i clear some space.