iPhone SE 2nd Gen: Odd Behavior of Storage Capacity under About section

iPhone SE 2nd Gen: Odd Behavior of Storage Capacity under About section

At least twice per month, I manually (by selecting and deleting)  clean out the storage on aforementioned 2nd Gen iPhone SE.  This includes videos, photos, notes, iMessage content, etc.  I also clear internet history.

For iMessage and Photos/Videos, I also remember to empty their respective recycle bins ('Recently Deleted' folders).  Just in case someone asks if I remembered to!

Naturally, I would expect the 'Available' storage field to keep increasing in value as I remove unwanted stuff from the phone.  Which it does, for a while.

Then, after a period of cleaning, I notice something odd: after a while, Available storage starts to go down!   As much as a half to one whole gigabyte.  

I will also occasionally do Volume-Up Volume-Down then hold down Power to do a forced reset.  Occasionally that will "knock some sense" into the About fields, other times, it does not.  

My question is: What else could be causing the Available storage field to go down, after going up as it should, as I remove stuff from memory?