iMac 27" Different Graphic Processors
I always use this site to give me an idea of relative speeds of graphics cards - http://www.videocardbenchmark.net/gpu_list.php [click the G3D header twice to sort by speed]

It's not necessarily any absolute indicator of the suitability of any given card for a specific purpose, but it's enough to give them some kind of recognisable hierarchy.

From it, you'll see that the M290 is considerably faster than the 755M, though neither is exactly stellar in performance.

As to why, only Apple can know that. They periodically switch between AMD/ATI &amp; NVidia &amp; have done for many years.