Seegee:
There are probably two things at work here. Serial as opposed to series.
Exactly. For "series," think of it like a TV series, that continues episode by episode until it is canceled (rather than until it reaches a narrative conclusion). A serial is simply a longer story broken into parts for easier digesting by the reader.
If I were writing a true serial (and I've only one one, a short one), I would finish it (or at least everything before the final edits on the later sections) before posting the first installment. It's one story, after all, and may require revisions to early sections in order to make later sections work properly. But that's just me. And as for a series, I'd just never do one - it's one sequel after another.