No idea why those specific settings are chosen, but my guess would be that it's a compromise setting between losing power to high-speed sync, but also having a shutter speed fast enough to freeze the action.
The D300's maximum sync speed is 1/250s.
The shutter speed in your camera is mechanically controlled by two shutters (front and rear), and the gap in between them as they sweep across the sensor. The faster your shutter speed, the smaller that gap becomes. 1/250s is the fastest shutter speed you can use with flash, where the entire sensor will be uncovered at one time. For faster shutter speeds, the gap will be smaller than the sensor, and it will travel across the frame. However, a flash burst is typically far shorter than your shutter speed, so if you're shooting above your maximum synch speed, you'll get black bands across the top or bottom (or both) of the frame, because only the gap between the shutters was illuminated by the flash burst.
Using FP mode puts your flash into high-speed synch mode, where the flash gives out timed bursts that match the shutter "gap" going across the sensor, so the entire sensor is still illuminated. But this bursting costs you power (and range) on the light, because the capacitor can only pull charge/current from your batteries so quickly. The faster you push the shutter speed in FP mode, the faster it's got to pulse, and the less power you've got to work with from the flash.
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