carbon copy cloner: doing full instead of incr. bkup each time?

it seems to me that my ccc bkup is doing a full backup instead of incremental each time. eg. it copies all my apps every time.
i've tried with two ext fw800 hdd, bkup up my 500gb internal drive... it keeps taking some 1.5-2hrs each time, day... what could that be? log file says nothing...

(latest ccc version on osx.7.5 on mbp8.1)