Sure I read this somewhere.
Tins of plum tomatoes must be made from the whole firm 'perfect' ones, so after processing, you get whole tomatoes out of the tin.
Chopped are made from the rejected, split, soft, over-ripe and oddly shaped plum tomatoes (probably hence the occasional bits of skin still left in).
Not really taste tested this myself but, for this reason (over ripe), the chopped are supposedly sweeter.