Authenticated Users' NTFS Permissions

This may betray my ignorance, but here goes:

My hard drive has two partitions. The second partition - D - contains only data files (documents, pictures, etc.)

My computer has 3 users - myself and two others. I am an Administrator and the other two are Standard Users.

The root of D has three folders:
D:\user1 (that's me)
D:\user2
D:\user3

The NTFS permissions on each user's folders are as follows:
Administrators - Full Control
SYSTEM - Full Control
user - Full Control

This prevents the Standard Users from accessing anyone else's data but their own, while maintaining the right of Administrators (like me) to access everybody's data.

So far, so good. Now here's my problem:

The NTFS permissions on the root of D (only) are as follows:
Administrators - Full Control
SYSTEM - Full Control
Authenticated Users - Read and Execute

If Authenticated Users does not have NTFS permissions on the root of D, I am denied access to D, even though I am an Administrator. If I double-click on the drive icon I get an 'Access Denied' message. However, if Authenticated Users is included in the NTFS permissions, I can open D by double-clicking on the drive icon.

Why can't I access the root of D as an Administrator ... why does Authenticated Users need permissions?