I would guess it's because you don't have your monitor set to millions of colors... but that gradient doesn't look any better than anything else I've ever seen. So again, I think AaronW pointed it out -- if you're getting waving in your artwork:
1) Monitor wacked out? If that gradient he showed you looks wavvy, then you either have a better monitor then my calibrated 21" Apple monitor here at work, or yours is not outputting properly.
2) you're not building your files right.
If you're using a gif, then of course, 256 colors may not be enough for the gradient. If you're using a png are you using 24 bit, 32 bit... hell, 48 bit? If you're using a jpeg, then maybe you need to up the quality of the output when you save it.
SVG and flash are vector -- they will obviously put out better quality gradients due to vectors type of file -- but a rasterized image can get darn close -- much closer than any normal person would arguably see.
Since you said Adobe + Corel -- does that mean Corel Draw, or Corel PhotoPaint (they also own Paint Shop Pro as of last week) and Photoshop or Illustrator... or InDesign or something else