Inconsistency in Invoice doc munber range

Hi People,

We have maintained invoice document number range as 5105600101 to 5105699999 and current number shown up as 5105600590 but when we check in the system we see that we have a invoice document posted with number 5105699999

in the system

I would like to know the below points

1. why is this inconsistency in numbering of invoice documents ?

2. How do i get to know what are the invoice numbers which went missing as I still wonder if the number range from 5105600101 to 5105699999 is completed for the fiscal year

My understanding:

System allocates the number to invoice documents randomly with in the number range and if we have a document with number 5105699999 it does not mean we have used up all the numbers - Am i right in saying this. - please confirm

Please give me some inputs

Regards

Pavan