If I had to guess I would say that the artists for the title cover and the manga are two separate people. Also, the artist of the PSP Cover art is called Alpha, but that's all I know.
Compare:
-Eyes in the manga are consistent: large black dot in the middle, while the PSP art does not have these dots.
-Curly's eyes are a much duller blue in the PSP are then they are in the Cover art
-The manga has much more defined face shapes for all of the characters, while the cover art's faces are very rounded, especially Curly's face.
-The earpieces in the manga are primarily white with a black ring at the base, like earphones. The earpieces in the PSP art are fully black.
-The manga art's shadows blend well and go from light shades to dark shades, while the PSP art uses grey as a shadow for more or less everything.
-The shading in the manga is used much more and creates a softer feel, while the shadows in the PSP art are hard - there might be a little variation such as at the edges of Quote's arms but shadows are generally much sharper.
-The writing on Quote's hat is jagged and red in the Manga, while in the PSP Art it is grey and straight, as well as both appearing different in general layout.
-Quote and Curly in the manga are clearly robotic, with panel lines on Quote's face, ridges and such on Curly's chest, visible joints in Quote's arm, etc. while the PSP art has them appearing almost human.