[CentOS] RAID question

Hello.

I have several systems (CentOS 3 and CentOS 4) with software raid and 
I've observed a difference in the raid state:

In CentOS 3 systems we find that mdadm --detail /dev/md* show:
..............
        Version : 00.90.00
  Creation Time : Fri Apr 16 14:59:43 2004
     Raid Level : raid1
     Array Size : 20289984 (19.35 GiB 20.78 GB)
    Device Size : 20289984 (19.35 GiB 20.78 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 5
    Persistence : Superblock is persistent

    Update Time : Thu Apr 20 15:52:50 2006
          State : dirty, no-errors   
<--------------------------------------------------- RAID STATE
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0


    Number   Major   Minor   RaidDevice State
       0      33        7        0      active sync   /dev/hde7
       1      34        7        1      active sync   /dev/hdg7
           UUID : 232294aa:45bd9dea:c62face1:2fbf7a60
         Events : 0.38
..........................................

while in CentOS 3 systems we find:
..................
        Version : 00.90.01
  Creation Time : Wed Feb 15 10:13:41 2006
     Raid Level : raid1
     Array Size : 76051584 (72.53 GiB 77.88 GB)
    Device Size : 76051584 (72.53 GiB 77.88 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 1
    Persistence : Superblock is persistent

    Update Time : Sun Apr 23 12:48:42 2006
          State : clean     
<-------------------------------------------------------  RAID STATE
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0


    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
           UUID : 35c88d0d:0a9b5ba5:78a32da7:5d5cb1ec
         Events : 0.251478

..................

Which is the difference between dirty and clean state? Both systems work 
fine.