Where as a regular kit comes with all the usual components to complete the plane minus the engine, radio, covering etc..
the short kit comes only with the needed precut components (ribs, doublers, etc...) and leave you the option to get the best quality wood you want to complete the kit. Stuff like sheeting, 1/4" balsa, etc... gives guys who really want more flexablity and control over the finished product to do what they want. A lot of companies use a heavier or lower quality stock in the effort to get more quantity out. Not to put the quality of the kits down, just the price of getting a good kit out at a decent price.