It seems they are linked by same category and same city.
Same category is not enough.
In category OGRODZENIA i have 4 ads and here I try to understand this issuu
By default, related listings are fetched from the same category. If you also installed Geolocation, then related listings are also filtered by location (only listings within the radius defined in settings are displayed). If you add more listings with the same categories and close location, related listings will be populated.