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.