I have solved the Great Word Count Widget Mystery. Sort of.
According to what I can gather from posts in this topic
on the NaNoWriMo Tech Help forum, the mini-widget
and the larger graph widget
each recalculate your daily word count minimum based on how much you've written so far. This is quite clever, and isn't necessarily a bad thing.
On the big graph, for instance, the gray bars, which represent the daily minimum, are shorter or longer depending on how much you've written. If you reach the green zone every day, you will end up with 50,000 words. Easy peasy.
Unfortunately, what remains a mystery (despite explanations in the above-linked thread) is how the mini-widget arrives at its goals, what the numbers mean, and how, exactly, they're supposed to be useful. Still, it makes me feel better to know that it doesn't, in fact, make any sense, and that no one else understands it either. Maybe next year someone will write a novel about it. :)