Device storage decreasing despite no new additions

I am very happy with my Q10. Awesome. But I do not understand why the device storage decreases when all I do is add photos and mp3 files to my 32GB microSD card.
Background numbers are : OS 10.2.1.2122 on Q10
Device storage free : 8.1 GB
MicroSD card space free : 23.6 GB
I have added ZERO new apps since the initial loading. The apps show no new size increase.
I have had no OS issues, or any issues at all, for that matter.
When I add multiple mp3s, it stores on the microSD card, and the storage measurement reflects that.
But it also decreases the amount of free storage in the device storage measurements.
I searched the forums for a direct answer, and found none. Am I missing a setting somewhere?
Thank you for a response.

As you open and close apps, the free memory decreases.
If you are speaking about actually STORAGE on the device memory, then I have no idea why it increases. It will as you get more emails, text messages, etc.

Most likely apps you are using are caching information. Have a look in device manager > storage and see what the culprits are. Some apps will have a clear history type function to help reclaim some space, others like the Crackberry app won't, and will need to be removed and reinstalled every so often.