It's common for bike lines to use the same frame, and just up-spec the components as you go up the line. For example, to use the Shimano line, Frame X with Taigra is entry level, Frame X with 105 is a step up (and a step up in price), Frame X with Ultegra and finally with Dura Ace. Major changes in frame (say from touring geometry to racing) will result in a new line.
I'm not familiar with the Giant line, but I'd venture that they do things similarly.
Fit and comfort and how it feels to you are the key things here.
