No matter the white SMD LED, IR LED or other colored LED. When we just put the LED directly on the circuit without any resistor or capacitor, people might find out there are different current that the LED will take even it's the same color or wavelength of LED on the board.
The short answer is: Voltage itself doesn't directly control the current. It's the difference between the supply voltage and the LED's forward voltage that determines the currect. And this current must be explicitly limited by a circuit.
Then why those issue happend?
We usually tell people that there is a range for the SMD LED or DIP LED voltage. For example, IR LED have a voltage on the range of 1.1-2.0V. Then it means there must have some LED got voltage on 1.3V or 1.5V. But they are all in the standard range.
However, for each single LED, they might got different voltage. When we just put it on the circuit directly, then some LED will got more current for lighting and some will got less because the voltage is different. Which will make some LED have higher brightness or radiation and some LED don't even got enough electricity to power the light.
In order to solve this problem, the end user and just put some resisitor on the circuit to limit the current. Or the manufacturer, which is us, the BEST LED factory will use tighter binning to make sure the LED got similar voltage.
