Why the membrane potential is negative, if there is a higher concentration of positive ions inside the cell than outside (counterintuitive at first) ?

Net membrane potential is equal to the difference of positive charge and negative charge existing at the inner and outer side of the cell respectively. Since outer negative charge is higher in concentration, the net membrane potential is negative

Explanation:

The cell membrane potential defined as the net difference of ion concentration existing at the inner and outer side of a cell membrane at nay instance.

When the cell is at the resting potential, the positive potassium ion starts accumulating at the inner surface of the cell membrane as it becomes favors positive potassium ion. This leads to development of negative potential at the outer side of cell membrane by the accumulation of negative ions or charges. Hence, a net negative potential membrane develops.