Yeah in this case between these 3 supermarkets there are 2 options:
1. You choose your general location or
2. You don't choose
In the first option you get one less general category to choose from (for example they may not have fresh fish)
As far as I can tell, in both cases the supermarket closest to the delivery address is responsible for filling out your order and usually what happens is that they call you to let you know that they don't have something and suggest substitutions.
Seems kinda suspicious to me.