3421070 - Sales Order Numbering Gap

You have noticed that when Sales Orders are created via web shop (web service) the system creates gaps in the numbering sequence of Sales Orders.

SAP Business ByDesign

This is the system expected behavior.

The reason the system is performing gaps within the numbering of the documents is because the numbers are stored within the internal buffering system. 
Within the internal buffering system, the engine selects the numbers randomly from the define range to create the documents, however it does consider a chronological order when executing such action.

This logic is applied for both sales orders created from the UI (user interface) and from web service. 

2675078 - Gaps in Number Range of Outbound Delivery Numbers

Sales Order, Numbering, Range, Gap, Sequence, Skip, Issue, Error, Web Shop, Web Service , KBA , AP-RC-NR , Number Ranges (SAP Business ByDesign) , How To