Hi Dim,
Its due to Number Range Buffering only, system reserves some places (Upto 10).
This is to avoid the situation in which some one else is creating the different BP simultaneously ,then system creates buffer so that same number is not assigned to two BP.
Normally BP Number Range is made external and synchronized with Customer
(BP is created as soon as Customer is created). This is the best practice.
Let me know if it helps you.
Cheers
Aditiya.