Is there any greater lie in this world than JAV covers?
Yes, but it still got the usual 20 or so pages that you'd expect from a doujin that are actually fully drawn, the rest are just preview for the following release http://exhentai.org/g/379419/d3604c6a61/
Usually when this happens it is because they ran out of time and didn't make it to Comiket day. They still need to bring something they declared they were going to sell to Comiket per rules, so they bring the roughs they have. Or they just want to sell a preview to fans besides what they already have.
Remember when /jp/ was hyped about Trio Side Dish?