Answer by Cort Ammon for Is spring force really a conservative force?

The position of the block is the same in both cases, but the position of the block does not wholey define the potential energy in the spring. The length the spring is stretched defines the potential energy (along with the spring's coefficient).

If you don't change the setup of the system, you can say that the position of the block defines the length of the spring, because you can find the length of the spring given any arbitrary block position. However, in (c) you change the system by dragging it around a nail. Now, instead of being a straight line between the wall and the block, the spring follows the two diagonal lines. The sum of the length of those two diagonals is longer than the length of the straight line, so the spring has more potential energy stored in it.

Another way you can see the same effect is if you were to start with case (b), and then use a rod to stretch the spring upwards, until it is just as in (c). Intuitively you will have to put work into that spring, so the system must be gaining energy.