Yep, Ari is correct. Just to clarify your initial question Andrea:
Is the open rate based on someone clicking "display images" in the email?
I am thinking that if they dont do that then Marketo doesnt know they opened it.
Correct! And in most email clients, images are disabled by default, so you shouldn't put too much emphasis on open rates for this reason. Click and open-to-click vs your industry/historic average are generally much better indicators.