I was mentioned in this message while I was not in the chat room. Usually, this should become a notification after 15 minutes.
However, I didn't get any notification as can be seen in this screenshot:
As you can see, I do get notifications from comments and from chat rooms in Stack Overfl...11
To use you as an example:
@TonyMeyer, i.e. an exact match will cause a notification if you've been in the room in the past seven days.
@Tony, i.e. a first name match (to be precise, a word boundary match) will cause a notification if you've been in the room in the past two days.
@Ton will notif...
Referring to: https://scifi.meta.stackexchange.com/a/939/31259
Point 3 looks like the answer to your question because you got pinged with '@ber':
@Ton will notify you if you've been in the room in the past two days and there's no other user for whom this would be a better match, e.g. someone ...