If you upscale directly; you're just making the pixels bigger...that's OK up to a point; but if you're not upscaling by a whole number then the half-pixels can look messy.
If you upscale past the desired result; then downscale to the desired size then you are (can be) ironing out some of the jaggies at the cost of some overall blurring. The downscaling bit hides some of the errors created by upscaling.
It's difficult to explain; but if you can be arsed to try an experiment then try this:
1) Take an image. Enlarge it x5 in your favourite graphics program. Save the image as an example of direct upscaling.
2) Take the same original image and upscale it 15x with Smilla Enlarger:
http://sourceforge.net/projects/imageenlarger/
....then downscale it 3x using your favourite graphics program
3) Compare the images. The directly upscaled image should be blockier; and the up-then-downscaled image should be sharper overall; but with a slight hint of blurriness.
Biting the hand that feeds IT
