That's the thing, it's 4X more pixels than 1,280x800, (not 1440x900) so it scales perfectly. But with 1440x900 it doesn't scale out at all, so a single pixel might be shared across 2 pixels.
What was the old and new resolution? I'm willing to bet that it scaled out.
EDIT: I looked it up, the 3GS had 480X320 res while the Retina jumped to 960×640. That's double the pixel width and height which means it's 4 pixels to 1, a perfect match and perfectly scalable.
So that makes perfect sense, just like using 1,280x800 resolution on a 2,560x1,600 display. What I can't figure out is how you use 1440x900 with clarity.