Firstly, it's clear that some picked additional kits without a Core kit because the total number of orders is much higher than the number of Core kits ordered.
As you point out, more Core kits were sold than others, which means many Core kits must have been sold alone. I suspect it's actually much more than just the 78 kits difference between the sum of the two most popular additional kit options which were sold alone, and that a minority of users purchased multiple additional kits.
In the cart system, if, say, you purchase both Novelties and Extension kits, then only one of them can be added on to the order of your Core kit. For the other, you must add a second order, which will have no Core kit, but will have the other additional kit.
There is also nothing in the cart system to stop you from ordering all three of Core, Novelties, and Extensions as separate orders within your purchase. Some people may prefer to do so if they want different quantities of each.