Ssanguanini your statement fits the Netatmo shop but not necessarily the products offered here.
21043024 is from the shop "Long Island GmbH". I cannot understand how this pricing came about.
The other two products are both from Digitec, so I assume that they are different deliveries of the same product and that is why the price difference has arisen.
In any case, I can't find any indication in the product description that this is the bundle sold on the Netatmo website.

Inexpensive variant Power supply directly at the old doorbell
Variant with mains adapter if no power available
Variant with alarm siren most expensive