This is a steemd.com bug. I was using some numbers that I thought wouldn't change, but it looks as though the weight of each vote lowers over time. So a weight of 0 means it was either a negative vote OR an old vote. I will have to find another way to get this data... in the meantime, I've removed the thumbs-down icon. The thumbs-up icon continues to represent upvotes, but only recent ones.