It's likely due to two (or more) automated pricing scripts duking it out. Here's how it works:
Seller 1 is a well respected seller. They've got thousands upon thousands of good reviews. As a result, they never price themselves the cheapest, but maybe pretty close to that. Their script finds the cheapest price, makes sure that it's above their cost, and then prices it at 15% above the cheapest.
Seller 2 is not as entrenched in the market. They try to undercut their competitors. They have good reviews, but not as many reviews over all. So, price is their main weapon. Their script seeks out well reviewed sellers and prices at 5% below their competitor.
Both scripts update every 15 minutes or so. You can see where this is going if both scripts don't have an upper bound. Each script is designed to make the most profit, and not be the lowest by too far. For example, if pencils suddenly had a shortage, you'd want your prices to automatically raise to match the perceived demand. You wouldn't want to be the sucker who sells all his pencils at a tiny profit while everyone else gets far more money.
Or, you know, it could just be a data input error. :p
