Oda says that when he was a kid reading Shonen Jump, most of the stories tended to be inconsistent. He liked Kinnikuman around that time.
During that period of Jump, they'd introduce a new character(s) in a big fashion one week and the next week, those characters would quietly be phased out.
Mangaka prioritized writing a fun and interesting story above all else and thus consistency took a backseat.
Kids also didn't really care about just issues and would read it regardless so when Oda decided to become a mangaka, he decided he would make everything consistent.
The past and future and all characters would be connected and wouldn't be phased out like in the past. One Piece represents this objective.
However, currently he says that people have become rather nagging about manga. They will look through the various inconsistencies or contradictions with a more fine tooth comb.
He said that in the past Shonen Jump was read almost exclusively by kids but now adult also read jump so that may be one of the reasons for this "new" nagging.
Now, Mangaka, in order to satisfy these older readers, have to make sure everything is correct and doesn't become inconsistent.
Oda himself feels slightly responsible for this. However, when creating a story, making sure everything is connected or all the events have been properly foreshadowed of hinted; "rules" like these do not exist.
He says that, as a challenge, he created a story where everything is consistent but normally you can freely create stories without having to do all of these.
Stories like "the nutcracker and the mouse king", which were created to entertain people are born through this "free" writing.
http://www.arlongpark.net/showthread.php?t=42229&page=42&p=3791544&viewfull=1#post3791544