Why passing heathcare is like catching a plane

So Obama's healthcare reform looks like it is going to pass. I'm more interested in the mathematics of it than the politics. How can it be that right up until the last minute there is such suspense as to whether it will get passed and yet at the same time a fair certainty that they're going to get their 216 votes? It reminds me of the way I am late for airplane flights. I've never been so late as to miss my flight (touch wood) but I have been repeatedly late enough to have to break into a run. Why is that? You'd think that statistically speaking it would be random: I'd miss it as often as I didn't and by different amounts each time. And yet I always make it with just minutes to spare. Clearly my efforts to be on time increase, incrementally, the closer I get to being irreversibly late. The same law is obviously at work in the legislative process, even though it involves a body of people: the very difficulty that creates blockage, creates the antithetical spurt of energy required to remove that blockage. It's great narrative: it gaurantees both nail-biting suspense and its successful resolution, every time.