The documentation on DriveInfo.TotalFreeSpace and on DriveInfo.AvailableFreeSpace tell you respectively:
Remarks (
TotalFreeSpace)This property indicates the total amount of free space available on the drive, not just what is available to the current user.
Remarks (
AvailableFreeSpace)This property indicates the amount of free space available on the drive. Note that this number may be different from the TotalFreeSpace number because this property takes into account disk quotas.