I have an 8bg iphone.. it says I have 6gb available yet somehow I dont have enough space to do the 660mb update on my phone.. WHATS THE DEAL... wheres my storage

I have an 8gb Iphone 4.. I know stupid me for not getting more storage/memory.. It currently says I have 6gb usage on this memory.. but somehow I only have 330mb of storage available.. Im trying to do the current update and need 660mb.. and even though I should have 2 gbs of available storage I DONT.. currently fuming.. and to get any answers from Apple I need to pay a 19$ incident fee to get an answer.. so irritated.. can anyone explain whats going on..