Are you perhaps exporting with printer marks or slug or bleed?
You may be seeing a rounding error. Set the measurements to millemeters.
It should be 210x297.
Thanks Bob! I did that, and it made no difference but have now sorted it somehow! I think it must have just been an odd glitch, the export process adding on what would have been a 3mm bleed...even though the settings didn't have that selected and the document wasn't set up with a bleed originally! So when i checked the 'use document bleed settings' it worked fine as there was no bleed. Anyhow, not sure if that's even possible but it seems to be ok now! Thanks for your reply, great having such a swift forum network!
And thanks to Grant to for your reply - i had all those settings unchecked but as you can see above, it has somehow sorted itself!
not a glitch...
if you changed the settings for a profile on export eg: changed bleed... and create a new doc, export to PDF: the modified settings will default. So, if you start a new doc and export to pdf and you see the preset eg ([high Quality print]) with (modified)) next to it: either select the default profile or go through the settings to make sure you happy.