Why is the potential energy of a spring $U=1 / 2 k\left(y-y_{0}\right)^{2}$ instead of $U=1 / 2\left(y^{2}-y_{o}^{2}\right)$, when a change in kinetic energy is $\Delta K=1 / 2 m\left(v^{2}-v_{0}^{2}\right)$ and not $1 / 2 m\left(v-v_{0}\right)^{2} ?$