Python's sqlite3.register_converter() does not convert double to Decimal

Question

In sqlite3 there is this table:

create table t(d double)  

In python, the code is:

sqlite3.register_converter('double', Decimal)
...
for d, in connection.execute('select d from t limit 1'):
    print type(d)

The printed result is: <type 'float'>
Any idea why?