I also own a Skoda Octavia 3 FL powered by 1.5TSI 110kW (DADA) 6MT and a Golf 8 powered by 1.5TSI 96kW (DPBA) 6MT, and the consumption numbers on the same stretch (~700km) and speed (130km/h) are 6.5-6.7L/100km for the Skoda, 5.8-6L/100km for the Golf (30kg heavier than the Skoda). Based on these numbers I think the Ateca numbers you mentioned are a little off.
Regarding the difference between the Ateca and the Formentor: first of all the DSG gearbox is different on the Ateca vs Formentor on the 1.5TSI engine. The Ateca has the the dry clutch DQ200 (one oil pump and ~2L of oil) which is more efficient than the wet clutch DQ381 (two oil pumps and ~6L of oil, 20kg heavier). Second the Formentor has quite an irregular shape, large wheels, and creates a lot of drag at high speeds. The Ateca seems like a more aerodynamic design. I did search for the Formentor official numbers for drag to confirm these assumptions but I couldn't find any. Still, the numbers seem a little on the high side. I'd like to find a Tiguan 1.5TSI DQ381 to test on the same stretch
Regarding the difference between the Ateca and the Formentor: first of all the DSG gearbox is different on the Ateca vs Formentor on the 1.5TSI engine. The Ateca has the the dry clutch DQ200 (one oil pump and ~2L of oil) which is more efficient than the wet clutch DQ381 (two oil pumps and ~6L of oil, 20kg heavier). Second the Formentor has quite an irregular shape, large wheels, and creates a lot of drag at high speeds. The Ateca seems like a more aerodynamic design. I did search for the Formentor official numbers for drag to confirm these assumptions but I couldn't find any. Still, the numbers seem a little on the high side. I'd like to find a Tiguan 1.5TSI DQ381 to test on the same stretch