I though because of the diffences between the timers used in analogWrite() depending on the specific PWM pins that the default PWM switching speed on a 328p based board was:
Arduino Pins 5 and 6: 1kHz
Arduino Pins 9, 10, 11, and 3: 500Hz
That's from an older source, but think it still applies that there are two different switching frequencies used depending on the specific pwm output pins being used.
Lefty