why python sqlite3 convert str to number by default?

sqlite_unicode.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import sqlite3
import sys

con = None

try:
    con = sqlite3.connect(':memory:')

    con.execute('create table tbl (val string);')
    con.commit()

    l = [tuple('%d' % s) for s in range(10)]
    print l
    # [('0',), ('1',), ('2',), ('3',), ('4',), ('5',), ('6',), ('7',), ('8',), ('9',)]

    con.executemany('insert into tbl (val) values(?)', l)
    con.commit()

    cur = con.cursor()    
    cur.execute('select * from tbl;')

    print cur.fetchall()
    # [(0,), (1,), (2,), (3,), (4,), (5,), (6,), (7,), (8,), (9,)]

    cur.close()

except sqlite3.Error, e:
    print "Error %s:" % e.args[0]
    sys.exit(1)

finally:
    if con:
        con.close()