Actually, early production materials had him be Prime's brother, but this was downplayed in the film.
Keep in mind, most video game/comic book/etc adaptations, prequels and whatnot were working off earlier scripts, prior to the movie finishing its shooting and making it through the cutting room floor.

 

Step 1: Optimus Prime (disambiguation) - Transformers Wiki
Step 2: Megatron (disambiguation) - Transformers Wiki

You're welcome.

 

This i would have to agree with. When they often refer to each other with the term "brother" it basically means they came from the same creator, they don't necessarily mean that they are brothers by blood/relation or anything. So you must take it with as little grain of salt as you can.

As for for Armada don't link any of the actual facts to any other series that you have watched unless its Energon and Cybertron.

 

It's easy to see how that weirdness confuses you - it was a big fat dubbing error that was created when the Japanese version of the Energon episode featuring this scene was translated into English, which someone added to the article in a moment of unhelpful literal-ness. The Transformer who left with Rodimus was NOT Megatron - the dubbed episode mistakenly referred to the leader of a group of Decepticons who left Cybertron with Rodimus by that name. It is so very wrong. Ignore that mistakes, and you'll do fine!  

 

Shall I remove it from the Megatron article, since it's so unhelpful?

 

Ah, ok. Thanks!