Antialias problem

[Egg Bramhill]

Coma

I think when you export a selection, then the top most and left most shapes determine the origin. In your file these wheren't exact pixel positions, so this created the offset in the two rectangles you had.

Try it.

Select all your shapes (except the 800x600 white rectangle) and export and you get the ghosting you found on your original.

Now select all including the 800x600 white rectangle and the ghosting dissappears. This is because the White rectangles origin is a 0,0

You can get the same effect if you choose export Area to Save / Page. This is probably your best option and should help you get the effect your after.