Why if the truss is statically determinate, both bars are free to lengthen or shorten?

In an indeterminate truss, the strain and geometry of the members play a significant role. So if we make one shorter or longer the balance of geometry and the distribution of forces will drastically change.

imagine a cantilever beam supported by a prop at the free end. Depending on the length of that prop the cantilever beam can be:

• Partially supported

• Not supported at all, even being pulled down,

• Or the prop is so tall that it not only cancels the loads on the beam but pus the beam under prestress.

But in the truss in your figure, if you increase the length of one of the members, that node moves accordingly, up, down, or to the side. there is no resistance from other members to the movement of the node, they can guide its movement though. of course, if you change the geometry you have to calculate new forces.