Possibly, since you did not have a signature, the option to include your sig with your post was not checked when you made those first 171 posts. Now that you have a signature, the option will be checked only in the new posts. If you go back to one of the old posts and look at
--Show your signature
.. in the advanced edit screen, you will see if the option has been checked.
[aside]this thread probably belongs in Website "Suggestions & Feedback" So I'll report it for moving.[/aside]
@ Wim Sturkenboom, the Signature is linked to the profile because if you change your sig, it will be changed in all previous posts too
(My current Sig is not my Original sig, yet it is still the sig shown in my first post on these boards!)