What is going on when I sync my iPhone5?

Ok so I have no idea what is going on with my iPhone recently.

I have an iPhone 5 that I've had for a year so it's got over 1000 pics, a bunch of apps and songs, etc like it normally would but a few weeks ago I noticed while syncing it said it only had 5GB of space left so I thought 'oh I'll just select a bunch of songs I want to keep on it and take off the rest using the 'selected playlists, artists, etc' thing in the iTunes sync settings thing when you plug an iPhone in', which I did and it then said I have 9GB now and 727 songs which is what I wanted to happen so thought it was simple as that and done.

Then I noticed a few days ago it says I only have 5GB again, I've only added 5 new songs, 1 tone and thats all, so that obviously isn't taking up all that room again. So I just now went through and plugged my iPhone back in to sync it and make sure all the right things were selected and deseleced to sync, then proceeded. It said I have 9GB back, but now looking at it after 5 minutes for no reason what so ever it now says I have 2GB left and 8.78GB of 'Other'.

So what exactly is going on?! What would be classed as other anyway? Please help!