As people have said economy of scale but also using cheaper parts helps out a lot. Giant I think are pretty good at cutting corners and using some pretty low grade stuff on a lot of bikes. One big way a lot of manufacturers are doing this and Giant is no exception is house brand components which can sometimes be OK but usually are cheaper and heavier.
As far as what @
Russ Roth was saying: Anyone who lists bike weights is generally lying to you in some way. Usually the bike is missing some key components to make it functional or it is the lowest weight they were able to get it and production is not near it. Now some companies are relatively close but I don't trust much unless I put it on the scale. Rodriguez Bicycles out of Seattle actually had a great story about that:
https://www.rodbikes.com/articles/we...-a-minute.html. Always makes me laugh. Also they make a verified 13.5 pound steel bike which isn't too shabby.