The monster manual (p. 309) says
Shapechanger. The yuan-ti can use its action to polymorph into a Medium snake, or back into its true form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It doesn't change form if it dies.
What exactly is the word "statistics" referring to? Does it just mean ability scores? Or does it include stuff like max hit points, speed and armor class? In what way (apart from narrative) do the two forms differ, because if it retains all its statistics then mechanically there's no difference, right? Or am I missing something?
Also, I'm assuming "medium snake" refers to the "giant poisonous snake" Stats in the monster manual because it's a medium beast, right? If so, what stats does the Yuan-ti Malison get from the giant poisonous snake if it retains all of its original stats?