w3 total cache generates files but

Hello there,
I tried supercache but then, because of unsolving problems (it didnt generate any cache file) i moved to w3 total cache.
I set page cache on and on mode disk enhanced.
Then, if i check the w3tc’s folder pgcache i found many (NOT ALL) posts.
First of all I dont know why it generates cache files only for some posts.
Then, when i see firebug’s log, i never see 304 but always a 200 response.

p.s. sorry for my english