Your second diagram is deceiving because pointing to a single point, and saying "N volts" is basically meaningless by itself.
For voltage to make any sense, you need to specify two points. A voltmeter (for example) has two inputs, and you connect one of those inputs to each of those two points.
Now, it's true that in many circuit diagrams you'll see things like
3V3, or things like that. When you see this, however, it means there's some implied reference point, usually marked as the ground. So, a marking like "5V" really means "this point will be at 5 volts above ground".
Here, the point you've marked as "V=15" depends on what you choose as the reference point (the ground, if you will). For the moment, let's leave the resistor out of the circuit, since it's (mostly) irrelevant to the question at hand. Instead, let's consider something like this:
simulate this circuit – Schematic created using CircuitLab
If we measure between A and B, we'll see 10 V. If we measure between B and C, we'll see 5 volts. If we measure between A and C, we'll see 15 volts. The voltage between B and C makes no difference to the voltage between A and B (and vice versa).