Glycemic index is just a bit weird in that it's a measure using 100g of whatever food you're eating. So even if something's made with straight glucose, if there's quite a few other macronutrients, water, etc. in the product then it's not going to rate quite as high.
Same thing goes the other way around as well. Table sugar's got a huge GI, but there's no way you'd take in 100g all at once (I hope).
Try looking into Glycemic Load, it's a bit more realistic. Plus if a food's label straight up says that it's made with a ton of simple sugars you really shouldn't need a glycemic index score to tell you that it's going to shoot your blood sugar up quite a bit.