They’re probably scheduled to come out at some point in the future. For some material, the studios will change their release plans. A movie scheduled to be released on DVD in summer of 2007 might be delayed due to music rights, for instance.
Actually, it is an official stated policy. Read the Terms of Use on the website, particularly the paragraph beginning with “In determining priority for shipping and inventory allocation, we may utilize many different factors, including without limitation, the number and type of DVDs you rent through our service, the subscription plan you select, as well as other uses of our service by you. For example, if all other factors are the same, we give priority to those members who receive the fewest DVDs through our service.”