Altering the Registration form doesn't affect the form in the modal.

I can alter user_register in my own module using hook_form_alter. When I go to user/register, I see that my alter is working. If I load the user_register form using Dialog, then I get an unexpected set of fields. Firebug reports that this is still user_register, but the username and email address fields are the same as in the user/uid/edit form (user_edit_form).

Basically all I'm trying to do is make sure that my modal user reg form only has two labels, two fields, no descriptions, and a submit button.

What is going on here? It's like Dialog is grabbing the wrong form, even though I can see in the page source that it has the proper form ID.