Ask HN: How does my email tracking service know WHO opened my email?

I've been using an email client that has an email open/click tracking feature for a while. I like to track opens for certain types of emails like sales or invoices.

Somehow, when I send an email to multiple recipients using either multiple TOs, CCs, or BCCs (or a combination thereof), my tracker seems to know which individual opened the email each time is it opened.

For example, I sent an email this afternoon to one TO recipient and two BCCs. One of the BCCed parties opened the email and the tracking software was able to determine that it was that specific BCC recipient rather than one of the other recipients.

Since the tracking is achieved with a hidden image file and the email is one single entity, how is this achieved?