When you make modeling chocolate from scratch, the corn syrup serves to weaken chocolate's bonds so that the chocolate takes on a more doughy consistency. Milk and white chocolates, since they contain dairy, are already soft to begin compared to say a bittersweet chocolate, so those types require less corn syrup.
Also it's not just the type of chocolate but also its percentage of cocoa butter (or lack thereof) that effects how much corn syrup or glucose is required in a recipe for modeling chocolate. That is because cocoa butter makes chocolate molecules stronger. So high quality brands of chocolate with high percentages of cocoa butter require more corn syrup in order to soften up into a pliable consistency whereas imitation brands like candy melts, which contain hydrogenated oils in place of cocoa butter, require less corn syrup because they already have weak bonds.
Unfortunately, there is no way of knowing just from reading a chocolate label how much cocoa butter chocolate contains (although you can always look to see if and where it appears in the ingredients list to get a sense of quality) so this is one of those recipes that you may need to tinker with before you get the desired results.