Because they install two 4GB sticks of ram to reach a total of 8GB. It's important to make this distinction because 2x4GB is faster than 1x8GB, but it doesn't allow for a trivial upgrade by just sticking in another stick as you'll probably need to replace both instead.

Good catch. In 20+ years of looking at configs online and in print, this is first time I've seen the total amount not presented.

Possibly dual channel RAM so 2 x 4GB is faster than 1 x 8GB in that case and cost-wise is probably the same.