I hadn't thought of that, actually. I'll check if it is. But I would rather get a reply from someone that works on the database. Matt would definately know that, but Wayne or Kevin might also know the reason.
I've done a lot of work on improving the PHP code associated with articles on SitePoint over the last couple of months.
As LuZer guessed, it's the number of paragraphs. Although each article is stored as one long block of text in the database, it gets broken up into paragraphs by article.php3 before being displayed. The main reason for this is to let us associate an image with a paragraph (to have it appear in the margin)... although this is a capability we don't use too often.
I get it now! An easy way to find out how it works is to go to an article page and increase the pid=number to one number higher and notice how the higher the number is the more text is 'eaten up'. It was all so simple!