In 星のカービィwhy is Kābī written in that way?

Maybe because they are using a modified form of the Hepburn romanization

As you can see here: http://en.wikipedia.org/wiki/Hepburn_romanization

In most cases, the long vowels are marked with an macron.

I´m not sure is this info in right, but...



From the Wikipedia.

Also, you can see this versión of modified Hepburn in the signs of the railway stations in Japan

For example, in the Tokyo Station, you can see:



Please, take a look at the roman versión of the name, just below the hiragana version.

In most situation, in Japan this version of the Hepburn is used

I´m not really sure, but is my best asumption. In fact, in my experience, this system of romanization is used in all the railways signals (and i see a lot of them)

No, I understand that. I'm wondering about the katakana?

¿Are you asking about the use of katakana?

In the official page, is wrinting as カービィ

http://hicbc.com/tv/kirby/

Anyway, is "normal" in Japan (as far as i know) the use of katakana in "strange ways". For example in japanese words to "highlight". Like que use of capital letters. Sometimes, even japanese make mistakes when use the katakana.

And have in mind one fact, they try to put in kana the "sound" of a word, and don´t to transcribe the letters.

I´m not really sure about the question in fact

Sorry to dig this one up but I asked my wife (who's Japanese) and it simply means a slightly shorter vowel sound with カービィ than with カービー。