The term "inactives" can be a little misleading, it doesn't always mean that they haven't done anything with your email campaign but for one reason or another they haven't been recorded as doing anything with it. This could mean that they haven't downloaded the images, clicked a link, etc and that could mean they just received your plain text alternative email (something that is sent out with every HTML email). This means that your open rate is actually your MINIMUM open rate and far more people could have seen your all important email.