I had this very same condition (HTC 8X) awhile back. I corrected it via a hard reset I believe...but then you've already tried that right? Maybe try a hard reset again? It only happened one instance...zero bytes showing for Apps even tho lots of apps installed. Coincidentally the overall storage seemed to add up correct if I remember right. I just remember it being a really wierd situation....but the hard reset resolved it for me (if I'm remembering correctly)
There might be a thread on this here somewhere reflecting other users experiences? Maybe a search would turn that up.
Good luck
Edit to add...
Ok found the thread in this same WP8 forum (and I had posted in it so that's why it was familiar lol) And yes a hard reset did fix it for me. From the thread's OP's screen shots It does look like the storage numbers don't add up (app numbers making up the difference?) contrary to what I remembered and stated just above here.
From April of this year (2013), here's the
thread: