iOS 11 differing storage showing

[Sheridan]

I've noticed on a few iPads (mainly since ios 11) that the amount of storage showing in Settings is always quite a lot more than a third party app shows.

 

For example on one typical device , Apple's settings show 6 GB free, three separate 3rd party apps all show around 1.6GB free - so who's telling the truth?

 

Or are Apple calculating the free storage in a different way (i.e removing the amount used by apps or something like that)