What causes USN value to increase fairly quickly?

I was just monitoring USN values using the repadmin /showutdvec dc1 dc=mydomain,dc=com and noticed that over a period of 1 hour and about 20 minutes the USN number increased by around 574 for DC2 and around 584 for DC1.

I have not added any objects or done anything with AD during that period.

Just users logging in or out of system (but less than 20 people).

I have never studied this before and was curious what would increase it as I have been digging a bit deeper into AD lately.

Repadmin: running command /showutdvec against full DC FCCDC01.MYDOMAIN.COM
Caching GUIDs.
..
3c5ea3e1-565a-438f-8606-ec5dbc83bf3d @ USN   3615746 @ Time 2011-03-25 00:03:29
53f57328-adc3-49fa-a10c-6772c6ff5082 @ USN   3080846 @ Time 2011-03-22 23:31:09
FCCSalem\FCCDC02                     @ USN   2487402 @ Time 2012-11-19 09:10:59
FCCSalem\FCCDC01 (retired)           @ USN   2551917 @ Time 2012-11-19 06:37:13
FCCSalem\FCCDC01                     @ USN   2560890 @ Time 2012-11-19 09:13:12

Repadmin: running command /showutdvec against full DC FCCDC01.MYDOMAIN.COM
Caching GUIDs.
..
3c5ea3e1-565a-438f-8606-ec5dbc83bf3d @ USN   3615746 @ Time 2011-03-25 00:03:29
53f57328-adc3-49fa-a10c-6772c6ff5082 @ USN   3080846 @ Time 2011-03-22 23:31:09
FCCSalem\FCCDC02                     @ USN   2487976 @ Time 2012-11-19 10:34:19
FCCSalem\FCCDC01 (retired)           @ USN   2551917 @ Time 2012-11-19 06:37:13
FCCSalem\FCCDC01                     @ USN   2561474 @ Time 2012-11-19 10:34:42

Thanks,

Greg