As mentioned above, the voltage follows Ohm's law, so depends on the current: U=R*I, and the currents in your circuit depends on the resistances. In other words, adding the parallel R6 will make more current flow through R4, but not through R5. (KCL) The current through R5 will in fact decrease. Hence the voltage will increase over R4 and decrease over R5, showing that the actual branch resistances matter in a divider.
More generally, in this case you have a current divider. Ohm's law is the law, and when you change one of U,R,I at least one of the others have to change as well. In a fixed-resistance device like a resistor, it's not going to be R, at least not appreciably.