Sorry, this is not manga, but I'd like to mention The Idolmaster.

The original game of The Idolmaster is an arcade game about producing girls as idols.

However, the first anime series Idolmaster: Xenoglossia is a robot anime. Girls ride big robots named "IDOL".

EDIT: Add some background of my understand.

Mushi Production

Mushi Production is a one of first animation studio in Japan. Mushi Production is known as Tetsuwan Atom which is first Japanese anime series. Mushi Production was founded by Osamu Tezuka who is "Godfather of Anime" and the Japanese equivalent to Walt Disney. This studio was created by manga/anime creators(animator).

In Mushi Production, animator's opinion is most important and business side people doesn't have voice.

Lack of business sense, Mushi Production was failed in 70's.

Sunrise

Sunrise is spin off company from Mushi Production. The founder of Sunrise know why Mushi Production fail, then they decide a new policy that have strong business executive member and animator can't poke to business side. For example, animator can't became board member of Sunrise. In Sunrise, It move in the opposite way, animator's opinion is NOT adopted.

Sunrise had a big success in robot anime. For example: Gundam, Code Geass and My-HiME

Namco

In 2007, Namco (the game company that created Idolmaster) finished acquiring Sunrise and merged executive member. In that time, Idolmaster is one of most successful game in Namco. The executive give a order to animator team (who creates My-HiME) that create animation version of Idolmaster. Animator in Sunrise is a professional of robot anime, doesn't fit to idol anime, but they have a policy.

Then they created robot anime that use character of Idolmaster.