And what's the difference between a normal skinny guy and a core skinny guy
There is no difference, some retailers just don't know the exact name of the denim and end up screwing it up. There are certain pairs that contain the word "core," such as Red Cores. However, your basics NF pairs (fits) are Regular Guy, Slim Guy, Weird Guy, Skinny Guy, and Super Skinny Guy. The two that you mentioned are going to be the same fit. Did they have anything else in the name? Skinny guy comes in a lot of different types, i.e. Broken Twill, Elephant 1/2/3, Deep Indigo, Indigo/Indigo, etc... I'm sure the pairs of different from each other, but Skinny Guy is no different from Core Skinny Guy, the retailer just added the "core" in there by mistake.