Why are vaillant boilers lighter?

[MaxP779]

Why are these boilers lighter than the comparable worcester bosch models?

Looking at the ecotec 831 and the 29CDI there is a ~10 kg weight difference between them. I hear vaillant uses a stainless steel heat exchanger vs worcesters aluminium, and steel is heavier than aluminium so... i don't really get how they managed to shave 10kg off the thing.

 

[Nightglow]

Wall mounting plate I guess accounts for some of the weight, Valiant just uses a small narrow bracket to hang their boiler, whereas Bosch uses a complete full size wall mounting plate for mounting the boiler.