fiat stilo 1.9 jtd active (80)?

i have a fiat stilo 1.9 jtd active (80) turbo diesel and latley ive noticed the fuel consumption going down from about 46.5 to 40.0 does anyone no of a problem that can cuase this to do this or is it normal i dont drive the car harshly or fast so i dont understand why this would do this
 
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