When someone orders lets say 10 proxies, these 10 proxies are randomly put on the existing subnets. This ensures that all of the proxies are evenly split up on the existing subnets. I can ensure you that proxy location doesn't play any role if you accounts are in good shape.
I provided a little example here:
EDIT : This is done to ensure to safety of the accounts on these subnets. Splitting up the proxies to different subnets makes sure that there is no spike in accounts on one of the subnets - which is unusual behavior. That's also the reason why I don't sell a unlimited amount of proxies daily and why I don't offer a website for delivery. I am running my own accounts on the subnets, so it is in my interested to keep them from getting banned. Making profit with selling a proxy service is not a higher priority then keeping my subnet alive and healthy.