I think the difference here is that glucose can be converted to glycogen, but fructose can't. Both high-fructose corn syrup and sucrose are a combination of approx. 50/50 fructose/glucose (the most common HFC in use is a 55/45 split; sucrose has a chemical bond between the two sugars, but HFC doesn't). My understanding is that when most people refer to sugar, they are talking about sucrose, and not glucose in isolation; therefore, ~50% of 'sugar' can't be converted to glycogen.