I don't know the actual reason, you'd have to ask someone on the dev team there but here's my understanding.
Using right arrows to show a closed menu and a down arrow to show an expanded menu is a visiblity of system status indicator. It indicates that a group is either open or closed.
The android arrows on the other hand, are not meant to show the status of the folder (Open or Closed). They're meant to show the user that clicking on it will either open or close the folder. They're action buttons. The down arrow indicates that this group CAN be clicked on and something will go down, and when open the group can be clicked on to close that group.
You can change the arrow's direction if you'd like to follow conventions.