128GB reporting as 16GB

I recently purchased a 128GB Samsung Galaxy S6 (SM-G920I) to replace my rapidly dying S4. I used SmartSwitch to transfer most of my information from the S4 to the S6.
First thing I did was flash the official 5.1.1 firmware for it from the SamMobile website. (firmware G920IDVU2COGA_G920IXSA2COGA_XSA as I am in Australia). I then rooted the phone using TWRP 2.8.7.1 "zeroflte" and the "UniKernel 6 for TWRP". It all seemed to go well and I then used Titanium Backup to restore my S4's backup apps and system settings.

But now I have noticed something weird. If I go to "Settings -> Storage", the phone reports its storage as 16.00GB; but if I use my File Manager it reports 114.0GB available. And now Samsung Update tells me there is no more space on my phone (but Google Play quite happily updates *its* apps).

Anyone have any idea why the discrepancy? And how I may fix it?

Thanks.