Artichokes contain, among others, the compounds potassium salts of chlorogenic acids and cynarin, which are believed to contribute to this effect. Cynarin has been shown by scientific studies to have a perceived effect on drinks such as water equivalent to adding 2 teaspoons of sugar to 170ml of water. While this effect doesn't affect all people, a large amount of people can detect the sweetening effect after consuming artichokes. Likely it's a combination of both genetic responsiveness to the compounds and interaction of the just the right amount and types of compounds in the artichokes!