@Wookie
Hi!
Because of this:
http://chandoo.org/forums/topic/fill-down-a-formula-with-an-increment#post-150
Regards!
PS: It's very unusual that original posters come back to such old topics, usually you'd better start a new one and place a reference to the old if you consider it properly. But in this case you're asking about two specific posts of this topic, so it's ok.
Despite of this, perhaps you'd want to read the green sticky posts at this forums main page so as to know the guidelines that will lead to know how this community operates (introducing yourself, posting files, netiquette rules, and so on).