I recently found out that courgettes grow into marrows if you delay picking them.
(I was given a giant courgette/small marrow from a friend's allotment.)
i think they are usually different varieties, selected for different properties. Eg you can make a potato salad from your bag of baking spuds, but Anya might taste nicer.
I didnt realise that cross pollination of plants had zero impact on on the current fruit crop, but instead created Frankenseeds within.
Hence the warning that seeds you collect don't always grow 'true'.
My better half ignored that warning and ended up with a crop of large round green marrows with lovely bright orange flesh inside.
They were a hybrid of courgettes and giant pumpkins!
They were discussed on another thread.