What does Ghost count as an "Open"?

I was wondering if anyone knew what Ghost counts as an “open” for email newsletters.

I noticed my open rate seemed high very quickly after sending a newsletter. Checked the addresses and sure enough an email address I own (used to catalogue all the newsletters I send) was listed as an open. Not only have I not opened the email in that account, I haven’t accessed or logged into the account for some time.

Others who are listed as open don’t seem plausible either.

Is this a glitch or something in the way Ghost measures open rates?