Billing document number

Hi Gurus,

I have come across a strange billing number sequence in the system.

I have created two Debit Memos (DM's). The first DM  number 1354121304 (created on 13.12.2013) and the second DM created on cancelling the first DM and reissuing the bill has number 1354098464 (created on 16.12.2013). Now this is a little strange because in the numbering sequence the number of the second DM ( 1354098464 )  is a smaller number and should come before the number of the first DM (1354121304).

I am using internal numbering and both the above numbers belong to the same number range interval.

Can someone please explain why this might have happened?

Regards,

Abhijit