Correlation between perceived sweetness and sugar content A couple of months ago I started to take note that the perceived sweetness of foods isn't always correlated with its sugar content. One stark example was yogurt, where yogurts that tasted mostly plain or even tart would sometimes have higher sugar content than yogurts that were appreciably sweet. 

Unfortunately, I don't have any great examples to show at the moment. 

I was munching on one of these: Kirkland Protein Bars which got me thinking about this topic again. The bars are fairly sweet, but the product claims to only have 1 gram of sugar and it doesn't have any artificial sweeteners.

I understand that serving size is a confounding factor, especially given that I don't have any good examples to show on hand, but from what I recall, the serving sizes for the yogurts were comparable.

TL;DR: is there a strong correlation between how our tongues perceive sweetness and the stated sugar content? Are there other ingredients that can contribute to sweetness without contributing to the net sugar content?
Different sugars have different relative sweetness (in %) (Elmhurst College):


Sucrose: 100
Fructose: 140
High fructose corn syrup (HFCS): 120-160
Glucose: 70-80
Lactose: 20


Relative sweetness of some non-sugar sweeteners (NutrientsReview):


Aspartame: 180
Acesulfam potassium: 200 (in some diet colas)
Stevia: 300
Saccharin: 400
Sucralose: 600


Plain yogurt (100 g) contains 4.7 g sugars, but mostly lactose, which is not sweet. (NutritionData, milk composition)

Kirkland Protein Bars contains stevia (300% sweetness) (link - read at the bottom of the Nutrition Facts), which explains its sweetness. Stevia is a natural non-sugar sweetener.

Perceived sweetness can increase with the food temperature (ScienceDirect) and salt content (a study in mice). Liquid and solid foods with the same sugar content can have different perceived sweetness (ScienceDirect). And there are sweet taste enhancers...(ChemistryWorld)