Yeah, I noticed that sometimes two very quick upvotes can rocket an item to the front page.
On the other hand, one of my recent ASK HN posts (http://news.ycombinator.com/item?id=1167298) got 7 or 8 votes in the first hour but never made it anywhere near the front page, which seemed a bit weird.
Since PG says the code hasn't changed, I'd assume the number of people online and submitting during the given period has a large impact on whether any particular story will hit the front page.