Question about NN by system

Hi,

I am wondering why if we have a NN by system for a doc type, why we can have the same next number more then one time (we type it)  because the company is different ?

Example:

Suppose in system 43.

We have doc type OP that are not set by company, only by system.

We create a purchase order OP for 09000 company and let the system generate the NN.  So we get number = 1.

We create another purchase order OP for 09000 company and let the system generate the NN.  So we get number = 2.

We create another purchase order OP for 45400 company and let the system generate the NN.  So we get number = 3.