The site does explain its methodology.

By default, it shows the costs when using the cheapest post-paid plan with at least 500MB allowance for at least 30 days – cheapest in absolute terms, not per GB.

You can toggle the option to show prepaid plans with the same parameters (at least 500 MB for at least 30 days).

But since it takes data from the ITU and not from the real market, the numbers do indeed seem inflated. One of the cheapest (in absolute terms) prepaid plans for mobile internet in Germany is AldiTalk at 4€/1GB/4 weeks, so that the pageload should cost even less than 1ct (0.0056 EUR). Similarly, your Telekom plans are for 4 weeks. Maybe they've excluded these plan because it's not for at least 30 days.