Very low statistics

Hi, I'm trying for the first time this module on site that has been running Simplenews for several years, although we never upgraded to Simplenews 2. This site is sending about 30.000 emails every month and we are using SendGrid as our external SMTP sender.

The point is that I installed this statistics module a month ago and the statistics records are veeeery low. On a newsletters sent to about 3800 addresses, SendGrid statistics reports about 1200 opens but this module only 97. We would like to use this module results since are more related to each newsletter, and SendGrid reports by days (and we are maybe sending 4 small newsletters on a day), but for now the results are not very encouraging. Any idea about why is reporting so low opens? Thanks!