They use a base female mesh and mod accordingly, FemShep, Liara, and any female pretty much use the same mesh. Something about marketing target groups. IE Shaak TI in Force Unleashed...
You are correct. This is not specifically Ashley's body. As far as I am aware, this body is also used during female Shepard's romance scenes. It's interchangeable so they don't need to spend lots of time creating individual meshes for each character, that would use up more resources and time.
The female bodies look quite strange in this game. Mainly the breasts and rib area being quite thin and small and then normal size or big at the hips and down.
They look odd because they have to follow the same animation rig that the male characters use, so proportions are slightly distorted to fit the male "skeleton" if you will.