The only thing I can think is that it means newer stories continuously start out with higher scores. So a top story last week might have scored 3000, an equivalently top story this week scores 3100, next week's will score 3200, and on to infinity.
Not sure if that's what it actually means or not, though.
I'm an idiot, I guess. I actually understood all along. I recognised that the initial score for new submissions would be monotonically increasing, but the graph's x axis was throwing me, by making it seem as though a single story over time had its score dropping, but that's not it. If it were flipped horizontally, I think that would be more intuitive, maybe, since it might capture "these later (further-to-the-right) bars are newer stories" instead of stepping back. Or maybe not, and I'm just an idiot. :)
It's right here in the code, with this odd fixed timestamp:
seconds = epoch_seconds(date) - 1134028003
Which happens to be December 8 2005 -- a few months after reddit began and probably the date this scoring system went into effect.
I don't understand why newer stories should keep having higher and higher scores, instead of old stories having lower scores over time.
Actually I'd rather have a consistent system that allowed fair comparison of stories irrespective of time, to allow for choosing the "top ten stories of the year" for example.
Not sure if that's what it actually means or not, though.