In simple terms: Voltage is not what kills an LED, it's current. That is why there is usually a current-limiting resistor in an LED circuit. An LED is a diode. It’s just a diode that happens to emit light. Like any diode, an LED will always experience the same amount of voltage difference (between 2–3 V, depending on the LED) regardless of the supply voltage of the circuit (assuming it is in series with a resistor), since it is a non-ohmic device and does not obey Ohm’s Law.

and, loosely applied: PWM (pulse width modulation) theory would suggest 100% duty cycle to be the full 18VDC of the matrix supply....but the duty cycle of the pulses in the lamp matrix are smaller (about max 12.5%) and thus the LED "sees" a much smaller voltage and current, on average.
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Something else in the circuit is limiting the current then. Keep in mind that these circuits ware not designed to operate with LEDs....you are just getting lucky without a current limiting resistor in the circuit. Driving LEDs directly from energy sources without any great knowledge of what controls the current and how much current flows, will sometimes work well enough in many cases, but in many cases at other times it may work inadequately, and may lead to destruction or early death of LED or energy source.

Most LEDs are capable of withstanding current transients that are several times higher than the maximum rated current. But, the exact amplitude that a particular LED can withstand is also a function of the duration and frequency of the transient. Beyond a certain threshold, which can be difficult to determine the exact value, a single-pulse event will lead to an immediate catastrophic failure of the LED.

High-current repetitive pulsing, may or may not result in an early catastrophic failure of the LED. Repetitive high-current pulsing may result in a shortened life expectancy for the LED compared to the usual expected lifetime, on the order of tens or hundreds of thousands of hours. A particular device subjected to repeated transients at an amplitude some percentage above the data-sheet limits but below the threshold required for single-pulse failure will still eventually fail.

Your LEDs are working, at the moment....but may not last the normal life expectancy.

Why not just put in the correct resistor for your leds and not worry?

http://led.linear1.org/1led.wiz

http://led.linear1.org/led.wiz