Hi Arun,
Usually when the Accounts are subjected to server round trip and not saved but the Numbers are generated. These Numbers stay used from the number ranges. That is the main reason you might see that the Account numbers are not in sequence.
Because you never know who are all creating the accounts. Some times after the number got generated if we cancel the creation, that number remains used up.
I hope i have answered the query.
Regards,
Venkat