Moon Patrol was an arcade smash hit from Williams (licensed from Irem), so it was only natural that Atari wanted it on the 2600. However, it was around this time that Atari lost faith in the abilities of it's own internal programmers after numerous questionable conversions, so they contracted Moon Patrol (and several other games) out to a company called GCC. GCC (which stands for General Computer Corporation), was a small company that got its start by selling illegal speed up kits for Missile Command machines. After Atari sued GCC they reached a legal settlement in which Atari got Food Fight and Quantum (two arcade games GCC developed). After the lawsuit Atari and GCC started to work together and developed a close partnership, which included (amongst other things) developing 2600 games. GCC later went on to create the Atari 7800 before leaving the video game market, and is now a producer of laser printers.
GCC created a fairly faithful port of the arcade mega hit, but many people complained that the moon rover looked wrong. GCC must have been having trouble getting the shape right because they went through at least three different designs before settling on the odd shaped rover that we've all grown to love (or at least tolerate). Given a choice between the three designs the final choice was definitely the best, but still never looked right.
The most interesting thing about Moon Patrol is not what they left in, it's what they left out. Several features from the arcade were originally planned (and even programmed), but ultimately taken out of the finished game. For example, one of the earliest prototypes has the checkpoint letters and displays what area you're currently in, this was replaced with generic markers (small x's) and no indication of the current area. Why Atari decided to remove this feature is unknown, but the game would have been much better had it been left in.
Another missing arcade feature that was actually implemented at one point are the rolling boulders. In the arcade there are several areas where large boulders will roll towards the rover and must be shot. However when GCC tried to implement them they couldn't get the motion quite right and the boulders "bounced' instead of rolling towards the player. This may have been the reason they were removed, but they could have easily debugged them instead.
Implementing these two features would have made the 2600 version of Moon Patrol much closer too the arcade. It is unknown why these features were removed, but there was probably a good reason (memory constraints, poor playtesting, hardware limitations, etc.). However even with these missing features, the 2600 version of Moon Patrol is an excellent port and showed Atari that GCC knew what they were doing. While the graphics may have been a little on the odd side the gameplay was dead on, and in the end that's what's important.