There may be a relatively simple explanation for the differences found in the US study versus the Swiss study: cultural expectations. I lived in the USA for 27 years and found that for the most part people were rarely satisfied and rarely happy. The messages they receive are always the same: buy more, because more is the way to get happy, and if buying more doesn’t make you happy it’s because you haven’t bought enough yet!
But in Europe there’s not the same insane culture of perpetual acquisition. Thus it’s likely that Swiss shoppers were simply happy with their purchase (of jam, in this case) because they were regarding it merely as jam and not as a desperate but futile attempt to quell existential angst.