Apologies, I think I've found the reason:
"Other burners, that check disc ID codes, won't recognize a DVD+R DL disc because they don't have the DVD+R DL disc ID code in their firmware. Even when a disc has been recorded and finalized, other burners still look for a media code and generally will eject a disc without one. Sony has recognized this and will be making a firmware upgrade available in early May that allows Sony's 510/530 customers (500 most likely as well) to read/play a finalized DVD+R DL disc. Sony can't say how other manufacturers will handle this, however, we don't expect that many users will be playing back discs in other burners."