I’m glad you asked, since few subjects are more interesting that liquid sweetner viscosity — where pourability is a factor of the uniformity of molecules.
If you have a friend or family member who happens to be diabetic, you may have had occasion to see liquid glucose or even liquid fructose, which in their pure states are extremely runny things (as are pure tree syrups when you get them straight from the source). This has to do, as I said, with the uniformity of their molecules. Being mostly single sugars, they run out of a bottle like a load of ball bearings out of a cement mixer, to cite a highly improbably analogy. But just imagine if you were to add to that load of ball bearings a bunch of ten-penny nails, pieces of rebar, and lengths of chain, any of which could be up to fifty feet long. The whole mess would have a lot harder time exiting the container.
And so it is with corn syrup, which, even though it contains a very high proportion of simple glucose and fructose, is sold with a least a few of the long-chain starch molecules it was derived from left in it. This is what gives corn syrup the thick texture that’s so useful in the world of bakery. Specialty corn syrups can have such high proportions of long-chain sugar molecules that they barely flow at all (these are highly useful in candy making). It all depends on the types of enzymes that were applied to the original corn starch, and for how long.