uc_signup is incorrectly blocking email addresses across nodes

from what i can tell, uc_signup is incorrectly disallowing one email address from being used to register for one signup-enabled node if that address has already been used to register for any other signup-enabled node.

steps to reproduce:

1. create signup-enabled node N
2. signup for node N with email address E
3. create another signup-enabled node N2
4. attempt to signup for node N2 with email address E; Error: The attendee with email address ... is already signed up for this event
5. as an administrator, delete N-E signup via "Signup administration"
6. attempt to signup for node N2 with email address E again; no error