Here is why: Power = amount of work * frequency
To increase power, one could either increase the amount of work that's being done in a given time period, or have the same amount of work done faster.
In an internal combustion engine the amount of work translates to the torque and the frequency translates to the RPM.
For the laser engine, the amount of work would be the effect delivered by each pulse, or more precisely the push from the rapidly expanding air. The frequency is the amount of pulses per minute (essentially just like RPM, only it's pulses instead of revolutions).
Because the car is peing pushed by expanding air, the effect of each push will diminish when the car starts to move, because it's moving away from the push. In the power equation above, this means that the amount of work that's being done diminishes with speed, so in order to keep the same engine power, the frequency needs to increase with the speed of the car. So when stationary, an engine speed of 4k will deliver the same amount of power as an engine speed of 10k when the car travels at 330 km/h.
Now, if the engine is capable of a frequency of 10k pulses per minute, why not deliver all of that already from the start and have a huge power boost (2250 BHP?) at low speeds? I can think of two possible reasons:
1. Because it's simply been decided that 900 BHP is all you'll ever need.
2. Because it's possible that a higher frequency may result in a reduced effect. The car is propelled by expanding air, which means that the effect should be the greatest when the engine operates in cold air (since the temperature difference between the cold air and the hot expanding air is what determines the effect of each pulse). A too high frequency at a too low speed may result in the air warming up faster than new cold air is coming in, and the rise in temperature would reduced the effect. It's possible that the net result would still be increased power due to the higher frequency, but it would take more battery power and be less energy efficient.
When it comes to the sudden decline in acceleration when approaching top speed, it would be explained by the engine losing power. The maximum frequency of 10k pulses per minute is reached when the car does 330 km/h, above that speed it's not possible to increase the frequency any more and since the amount of work decreases with speed, the engine will lose more power the faster you go above 330 km/h. So when the car approaches top speed it's facing two obstacles: increased wind resistance and reduced engine power. This is what's causing the acceleration to decline faster than for a normal car, which only needs to fight against the wind resistance.
But why does the car have the same top speed even with a 50% engine limiter?
Because if you set the limiter to 50% you'll have roughly 450 BHP. The engine limiter does not reduce the amount of work that's being done - each laser pulse still packs the same punch. What it does is that it limits the frequency.
And because power = amount of work * frequency it will keep adjusting the frequency to deliver the 450 BHP. The unrestricted car hit the cap of 10k at 330 km/h, above which the engine lost power.
The restricted car will keep delivering it's full 450 BHP all the way until the frequency reaches the maximum 10k, which happens at about 400 km/h.
Remembering that power = amount of work * frequency and remembering that the limiter doesn't limit the amount of work, it means that when both the limited and unlimited car are operating at 10k, they're delivering the same amount of power. So at about 400km/h, the restricted and the unrestricted cars both have the same power. From then on, both cars lose power at the exact same rate until they both hit the exact same top speed.
