Don't pay attention to the stoichiometry of the pumps, that will only fool you! The charge is due to passive movement through open channels. This is shown as the "yellow tubes" with dotted lines in the diagram. Students of neuroscience are often confused by a very similar thing, as the resting potential of neurons (as well as voltage during action potentials, for that matter) is determined the same way.
The explanation is in the caption text, quoting from your figure:
Because the apical membrane is conductive primarily to K+, the apical membrane voltage is more negative than the basolateral membrane voltage, which is conductive to K+ and Cl-
The other missing piece of information is that the outside space is (relatively) low in potassium. That means that, given an available path for potassium to diffuse across the membrane, there will be a net flow of potassium out of the cell. That flow causes a negative charge to develop inside, because the potassium diffusing out is positively charged. Very few ions have to actually move to develop a biologically relevant potential (see explanations in other answers involving neurons: https://biology.stackexchange.com/a/76167/27148 and https://biology.stackexchange.com/a/57066/27148), so you can treat the ion concentrations as unchanged (and they will be, out to a fraction of a percentage point) despite this movement.
At some voltage, the negative charge inside the cell will counteract the movement of potassium, so it's not like this is a constant deluge of potassium flowing out, it's more of a maintenance trickle. You can calculate this "reversal/equilibrium voltage" using the Nernst equation.
When you have multiple ion species involved, the equation to use is the Goldman equation which is related to the Nernst equation but involves a relative weighting of different ions by their concentrations and permeabilities. As in the textbook description, adding chloride conductance makes the basolateral membrane less negative than if there was only a potassium conductance, because chloride moves out of the cell as the inside becomes more negative.