A simple question about SMS tablespace

In book TADM56, following sentences describe feature about SMS, "When an object (such as a table or an index) is created, a file is created in the directory. If an extent has been filled up, a file will be created in the next directory. This is performed in a circular mode."

I do following test:

CREATE TABLESPACE TESTSPACE

MANAGED BY SYSTEM

USING ('C:\DB2TEST\TESTSPACE1', 'C:\DB2TEST\TESTSPACE2', 'C:\DB2TEST\TESTSPACE3')

EXTENTSIZE 64

PREFETCHSIZE 32

CREATE TABLE TABLE1 (

ID INT,

NAME CHAR(10)

) IN TESTSPACE INDEX IN TESTSPACE

After that, a file SQL00002.DAT is created in directory "C:\DB2TEST\TESTSPACE3"

I don't understand this sentence "If an extent has been filled up, a file will be created in the next directory. This is performed in a circular mode". Based on the test I did above, how to understand the circular mode? Please advise.