this is the classic pattern that has been used by ecommerce sites for over a decade now. Once the checkout process starts, the number of links that may distract your checkout process drops dramatically.
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that would be the most logical explanation, but the article mentions that "The effect is smaller in Europe and especially large in India, but it does appear within each country, suggesting this isn’t the sole confounding factor." Which makes me think that there are other factors at play that add up.