I'm not sure what "opened" means as an event, but I've learnt to ignore it when compared to "viewed". I have had many clients say they haven't seen an offer to sign come through, even though the activity log has multiple pages of "opened". (perhaps their email provider programmatically opens and rejects it?)
And in the case they have 'viewed' the contract, i need to sift through pages of 'opened' to see what kind of activity they are actually performing.