One of the reasons behind the memory allocation is the 64 bit architecture and the other is that Sony usually reserves more memory in order to be able to cover all the requirements of a future android update.
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Got my 64GB XZ a week ago and it was quite shocking & disappointing to see that user available memory was only 50! That is ridiculously low considering this ain't my first 64gig inbuilt phone nor my first Xperia! Sony having toned down their OS, not sure what is taking up so much space. At a premium price tag, I'd expect better.