Correlation between perceived sweetness and 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)