Blurry pixels

[yeohray]

Some background: we had some designs created by a freelance artist using Illustrator. We would like to make some changes to those designs, but would like to use Xara Designer as it fits our budget and feels far more user-friendly (at least to us non-designers).

Here is the design when viewed in Xara Designer at 'Very high quality':



Note the two areas highlighted by the red rectangle.

Here is the design when exported to PDF/X-3, as viewed in Acrobat Reader:



Note that the left highlighted area is sharper than the right area, and is also sharper than the image viewed in Designer above.

At the printers, as they do not use Xara Designer, they had to import the PDF into Illustrator. Here is the design when viewed in Illustrator:



which looks pretty much the same as when viewed in Acrobat Reader i.e. the right rectangle area is blurry compared to the left.

However, when the 'frame' object is moved (upwards) away from the green area (highlighted by the red oval), we can see that the underlying image (the right rectangle area) is actually as sharp as the left rectangle area.



What is the cause of this? Xara Designer seems to export the green background just fine, but when it needed to fill in the green area in the frame object (the one containing the words 'Set 3'), it does so at a lower resolution?

Thanks in advance.