Although the membrane of a resting neuron is highly permeable to potassium ions, its membrane potential does not exactly match the equilibrium potential for potassium because the neuronal membrane is also

Slightly permeable to sodium ions.  

Explanation:

Resting potential is a phenomenon that occurs in our brains. It plays a great role in transforming information from one neuron to another neuron. During this process, such a procedure occurs. When nerve fibers become positively charged within the cells that is called action potential. This is the procedure of polarization in which positively charged sodium ions will transfer inside the cells. When depolarization occurs the slightly positively charged sodium ions will diffuse from inside to the outside of the cell membrane where this procedure occurs in high concentration. The resuming condition of negatively charged ions characteristics is called resting potential.