What are the possible causes of high fuel consumption in honda cars which are rated as the most fuel efficient?

Could need a tune up.

Are you talking about just one specific car or a whole model line. Honda has a very good AVERAGE consumption which means some models are better than others. True for Toyoto also, their Prius and high breeds have made their avergaes look much better. Drive a Sequoia on the road and you'll be lucky to get 18 mpg.

Heavy gas foot ;)