Have you ever noticed that some maple syrup grades feel thicker than others? Maple syrup viscosity decreases from golden to amber to dark. That means it feels thinner even though it is the same density.
In the table, flow period is the percent of the sap flow that season. But why does viscosity change? Just an educated guess but look at how the sugars change. Sucrose goes down and both glucose and fructose go up. Sucrose is a bigger molecule. Despite being microscopic, the bigger molecules could collectively feel more viscous. Glucose goes up from 0.06% to 0.48%. That still looks like a small number.
So if you've heard that maple syrup has a lower glycemic index than honey it's because there is less glucose. This trend through the season of increasing glucose also means that golden will have a lower glycemic index than amber or dark. When sucrose is broken into sucrose and glucose, the optical characteristics change. Plane-polarized light, when passed through a sample of pure sucrose solution, is rotated to the right (optical rotation, +65°). The mixture of fructose and glucose rotates the polarized light counter clockwise (-20°), thus the direction of rotation is inverted from right to left.
And that's why invert sugar is called inverted and why a blood glucose meter can be used to measure invert sugar. I haven't looked it up, but it seems plausible that lower invert sugar makes better candy and maple cream because it is more uniformly sucrose and has higher viscosity.