This might be related to a technical reason at database level. For instance when you sort domains starting from low bid to high bid, you don't want zero bids to be included.
I have a similar problem on my website with alexa ranks. Most domains have no alexa rank. When I sort according the alexa rank I want small numbers to appear on top because the lower the alexa rank, the better the domain. If an expiring domain has 12,314 alexa rank this would be very good for that domain. However, right now I have a technical problem and all those with no rank appear on top when sorted.