My Campaign Dashboard show different numbers than Smart Lists for that same email

Hi,

We set an email campaqign to our newsleter, and in addition we have created a smart list to locate the leads opened the email.
The campaign Dashboard statistics shows a higher number than the smart list we have created.
Dashboard resaults = 166
Smart lsit resault  =144
The smart list target "opened email" "newsletter email"

Any idea where that difference can come from?

p.s. already tried the "forwrd to a friend" filter as well.

Thanks