Are parents always to blame or to be praised for the way their children act in public?

My cousin has 5 beautiful little children ranging from ages 1 to 9. His kids are naturally very calm, low-key, shy, quiet and very much the way he was as child. When they are out in public they're all very calm and sit very still and are very shy and behave very timidly in restaurants clinging to their Mommy. We recently dined with him and his adorable family. Someone dining at the restaurant walked over to our table and praised my cousin for doing such a fine job raising his kids. The customer went on and on about parents not teaching their children manners and so forth and pointed out how well behaved these little children were. My cousin and his wife admit they are not doing anything differently than other parents and claim they are just "blessed" to have well behaved kids. His kids are just naturally shy and on the very quiet side. I think he could be right. What do you think?