Extras can come at random, sometimes even years after the series is considered complete by the mangaka. They'll suddenly be asked to do an extra for a new run of the tanko, or an anniversary, or the scan group will get lucky and suddenly get raws they'd previously been unable to find. Since they aren't considered part of the main story, the series can still be considered complete without them.
That makes sense. It still surprised me when I found out there was a new chapter. I don't think it's an extra though. I think they got lucky and found another chapter. :)
I didn't know they were going to add more chapters to this. I already read the whole thing, now I have another chapter to read. If it's not complete yet, they need to change it to Ongoing so people don't think it's actually Complete. I thought it was done. :O Now I'm going to read the new chapter.