In order for our system to report if a user has opened your email it requires a tiny image which, we embed in every email, to be loaded.
Some users have images turned off by default so unless they click on a link we do not actually know that they have opened your newsletter.
So, whilst your figures are accurate they can never be spot on due to the technicalities of the different email software out there.
If you are sending campaigns that include photos or other large images, here's a quick tip to improve the visual quality. SelectMailer will automatically resize images to match them to the width that is set in the template. We do a pretty good job of scaling images down, but as an automatic process, it is not always ideal. If you know that you want to use particular images, you can get better results by doing the resampling and scaling yourself. Then add the image at the correct size and avoid any compression from SelectMailer. When you need your images to look their best, custom resizing is the way to go.