Why does my phone show less storage than it's supposed to?
The flash chip inside your phone is 8gb, but not all of that space will be available to the user - some of it will be used by the other partitions, such as /system/ where the Android OS is installed, and the /cache/ partition which is used for temporary storage, among other things.

You can install this app to see how your storage is partitioned:
https://play.google.com/store/apps/details?id=com.cls.partition

For example, my Xperia SP also has 8gb storage

According to that app:


/lta-label - 16mb 
/system - 1211mb 
/cache - 542mb 
/data - 5500mb


The /data partition is the one you'll see as internal storage. 

Note that this doesn't actually add up to 8gb (more like 7gb) - the other point to remember is that device manufacturers always lie about capacities - e.g. my 16gb microSD card is actually 14.8gb. (It'll usually be a gig or so below what's reported, possibly much more for bigger devices.)