MDADM Array Being Limited to 2.0TB

I have an MDADM array with four 1TB disks in a RAID5 configuration. Here's the relevant part of mdadm --detail /dev/md1:

        Version : 00.90
     Raid Level : raid5
     Array Size : 2929537920 (2793.83 GiB 2999.85 GB)
  Used Dev Size : 976512640 (931.28 GiB 999.95 GB)
   Raid Devices : 4
  Total Devices : 4
Preferred Minor : 1
    Persistence : Superblock is persistent

It shows that the array is 2.8TB. Here is fdisk -l /dev/md1:

Disk /dev/md1: 2999.8 GB, 2999846830080 bytes
2 heads, 4 sectors/track, 732384480 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 65536 bytes / 196608 bytes
Disk identifier: 0xffffffff
Disk /dev/md1 doesn't contain a valid partition table

fdisk also shows it to be 3TB. However, df -Th does not agree:

Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/md1      ext3    1.8T  1.8T     0 100% /

Why does everything show /dev/md1 to be 3TB but the space is limited to only 2TB?

Other details: Ubuntu 10.10

UPDATE:

$ tune2fs -l /dev/md1:

tune2fs 1.41.12 (17-May-2010)
Filesystem volume name:   <none>
Last mounted on:          <not available>
Filesystem UUID:          1b4e9420-61ee-4ffd-817a-28831f2aeaf2
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery sparse_super large_file
Filesystem flags:         signed_directory_hash
Default mount options:    (none)
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              122068992
Block count:              488255968
Reserved block count:     24412798
Free blocks:              69746292
Free inodes:              121882953
First block:              0
Block size:               4096
Fragment size:            4096
Reserved GDT blocks:      907
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
RAID stride:              16
RAID stripe width:        32
Filesystem created:       Sun Oct 17 01:14:23 2010
Last mount time:          Mon Dec  6 01:24:31 2010
Last write time:          Mon Dec  6 00:45:41 2010
Mount count:              12
Maximum mount count:      23
Last checked:             Sun Dec  5 20:37:35 2010
Check interval:           15552000 (6 months)
Next check after:         Fri Jun  3 21:37:35 2011
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               256
Required extra isize:     28
Desired extra isize:      28
Journal inode:            8
First orphan inode:       55550208
Default directory hash:   half_md4
Directory Hash Seed:      2a386fcb-b24f-4f40-bf4c-7c03489b086c
Journal backup:           inode blocks

Did you add the 4th drive after creating the filesystem? That could lead to the FS being smaller than the underlying device.

Alternatively, maybe your ext3 has 1k blocks, as this would lead to a 2TB filesystem size limit (ext3 size limits).

Please post the output of "sudo tune2fs -l /dev/md1", and look closely at the Block size line.