Problem with string parsing

[Mike Howard]

If I do this:
string.lstrip('1_mature_dt=10-May-2002','1_mature_dt=')
I get:
'0-May-2002'
Which I did not expect
string.lstrip('1_mature_dt=20-May-2002','1_mature_dt=')
I get
'20-May-2002'
Which I expected
string.lstrip('1_mature_dt=01-Jun-2003','1_mature_dt=')
I get
'01-Jun-2003'
Which I expected

Is this a bug or am I doing something wrong?