Phone storage is low: 8.00GB / 8.00GB drops to 7.33GB when deleting two pics only

[p_p_s]

I am on Kit-kat 4.4.4 on a not rooted HTC Desire 620. It lags often, presumably because of space issues. About once per month everything is blocked and it shows:

Phone storage is low
Tap to make more space (tapping runs the make more space wizard)

When I run the wizard it shows the phone storage usage as 8.00GB / 8.00GB. When I delete only one or two pictures from the wizard entering Gallery, the usage drops to something like 7.33GB / 8.00GB although pictures are something like 2.5 to 3.5 MB only. When this happend the first time I think the used storage dropped even below 7 GB. (But some growth from apps was unavoidable.)

Questions:
1. Why that big drop?
2. If I now really have 650 MB available I would assume that to be OK for a responsive device. How much do I really have available?
3. How can I avoid again and again getting into a lagging state and to 8.00GB / 8.00GB?

I already moved all apps to the SD card that could be moved. I stopped all automatic apps updates and set all the bloatware back to their factory state.

 

[belodion]

I don't know why the big drop, but to keep the device usable you'll have to keep deleting things and clearing cached data like billy-oh. I had an 8gb iPhone that filled its storage over time and although it was possible to clear much of that by restoring from backups from time to time, it was so restrictive that in the end I just stopped using it.