If your situation is similar to mine, you probably have multiple email addresses showing on the contact record. Hubspot only emails the primary address. If you look at the list of subscriptions (scroll down to “[contact name] is opted in to [N] subscriptions”), note carefully which addresses are subscribed to which sub types. You’ll probably find that one of the non-primary addresses is subscribed, but the current primary address is not. Why and how that came to be is a matter for further research, to determine if something got mis-juggled during an update or if the contact simply opted out the current primary address from that particular sub type.
I hope that’s helpful.