Why would a video play fine on a 3G iOS device but not on a 2G?

I've had a contractor compress a video to be used 开发者_如何学编程in an app we're developing and while the video plays just fine on a 3G device, artifacts appear when watched on a 2G device. Why would this be? The 2G device is a 3.1.3 version.

Probably has to do with a mixture of network speed and hardware. Take a look at wired's comparison: http://www.wired.com/reviews/2011/02/verizon-iphone/

Same exact phone, but since ATT's network is faster, the Verizon phone plays a less quality video.

It's probably going to be difficult to get a good looking video over 2G. Also, is it even worth it? How many phones are still 2G? I can't imagine that many.