It's often the case that things eke in just under the wire. No coincidence that Beeminder was built to ensure that missing deadlines has a sting; we akratics often leave things to the last minute, and sometimes that's too late!
So you're right, that amount of cash is indeed at risk, but most of the time we stay on the road. Not least because we've got watchful eyes like yours to keep us on track.
You can get a sense from this forum thread how often derailments happen, and what the values paid out tend to be. Just like you, we're more likely to derail on a lower value than a higher value. (Fingers crossed that I haven't just jinxed the UVI commitment…)