Beginner Question about the logic behind Ohm's Law?
The calculation you're using is an approximation, but it's a good one. It assumes that the voltage across an LED is constant, no matter what the current is. This is not actually true, but it's close, because LEDs are diodes, and thus exhibit a characteristic diode IV curve (ignore reverse voltage for now):



Notice that as the voltage increases, the current increases very quickly. For reasonable currents, from the minimum operating current to the maximum current before the LED breaks, the voltage will remain pretty constant, +/-10%. If you have a constant voltage power supply, that means that the voltage across your resistor will also be constant, no matter the current, due to kirchoffs voltage law:







simulate this circuit &ndash; Schematic created using CircuitLab

You can see here that as we change the resistance, the voltage across the LED stays the same, so the voltage across the resistor stays the same. The current increases as we decrease the resistor.