OK, here's the whole story: when I went to Settings > General > Storage > Manage Storage (no iCloud is involved), again, when I added up all the details as indicated it didn't make 3.5 GB at all. When I opened iTunes I saw that the memory was inflated mainly by 'Documents & Data', hence I wanted to know what it is made of exactly. Please note that I reset the iPhone and the anomaly disappeared. But after a while (unfortunately I didn't realize when) it's showing now again a discrepancy between the total used and the details by apps listed. However, now in iTunes it shows 10 GB left and it's only on the iPhone that it shows 8.5 GB or so. Previously (before the reset), it was showing 8.5 GB on both my iPhone and on iTunes. I can't edit the title of my question now I guess, but again at present my question would be: what is the tool to see the very details of the storage used under Settings > General > Storage > Manage Storage (no iCloud is involved)?