no no.. the BMP and TGA formats have different methods for image compression. It's not A6 related. The BMP is a fairly old compression method. JPEG was then invented to compensate. However JPegs did not support animation. So, Gif was formed, but people didnt like how Gif compressed a series of images, so png was made. Also, each of these formats had their own variation of competition...

BMP, JPEG, TARGA, etc
Gif, png, (cant remember, but i know theres more)

The two formats look the same in an image editor because the both images are uncompressed in the program. What is being displayed is "Raw" data. The reason it uncompresses it is so that you CAN save it under another format.