"He felt like it was only necessary 13 pages."
Well, I don't think it is how this works... First, as professionals, Jump mangaka are used to draw the chapters with the 20 pages in mind, they do this every week, so they already think automatically what they will draw in a given week. Generally, it is best to use all the space you have each week and accommodate the story to do just that, so it is strange he would think that 13 pages was enough. Second, he is cutting his own payment with this since mangaka are paid by page drawn. Third, the magazine has a number of pages to achieve, some pages can be the difference between having a discount or not when printing the magazine, even more when it is huge number like Jump. That's why the magazine has generally 20 series with 20 pages each, with ads between each series and everything, it is planned in detail to reach the amount of pages required. When a manga has to have an unexpected break, they put an oneshot in its place to reach the number of pages. The same for volumes, they generally have the same amount of pages and many authors comments in their afterword in novels that their editors say that they have a x number of pages to make the afterword in a given volume. Anyway, in short, 13 pages instead of 20 pages can mess things up pretty badly so I don't think an author can make just 13 pages when he "feels like it".