Hi,
It seems that its standard not to generate the number of orders continuously because of number range buffering.
If you want to continuous numbers to be assigned then you need to deactivate the number range in buffering in your configuration. But doing so will have performance issues. Check with your fellow consultants
Check this link.
One Number in the Buffer - SAP Number Range Buffer (BC-CST-NU) - SAP Library
OSS Note - 456777 - Order numbers for internal number assignment
Regards
Vinoth