It is hard to answer this question without extra information...
1. It could be that it is a newer hub... which gets an explosion of traffic when it has first been published because it is easily found by members on hubpages, whether it is your followers or if someone is hub hopping or just going through the latest hubs published.
2. It could aslo be your keywords. Maybe you are using an excessive amount of keywords, or maybe you are using keywords that are never searched for. The stories are usually ranked by how well they are written and eventually, how well they will be found on Google. If you have poor keywords, it will lower the overal hub score. The only reason I know this is because I have gone back and worked on some of the keywords in my older hubs and suddenly their scores have jumped.
3. It could be that those that are sitting higher are a better topic. No body truly understands how the rating system works, but I do notice their is a trend in the articles that are higher scored, whether it is a better topic, better written, better keywords, or even longer in length.
Hope this helps!