Because unlike other smartphone tariffs, which advertise with a monthly contingent of data volume, Aldi Sud hides a hook here in the fine print. The 12 GBytes cover the entire duration of the tariff. That means you have 12 GB for all 365 days, If you surf excessively in the first month, things may run short in the remaining months.
However, little surfers should be happy about the offer. Because basically Aldi Sud offers here a tariff for about five euros a month, which can also be paid in full for a year in advance. In the package with a Samsung Galaxy A10, the annual package for 159 euros from 21 November 2019 is offered.