from this: http://wiki.asctec.de/display/AR/AscTec+AutoPilot the commiunctiong rate between PC and HLP can be 1KHz, but the UART baud rate is 57600bps,
a package may be about 100 bytes, so the rate is aroud: 57600bps/8/100 ,just aroud 70Hz,so how the 1KHz come from?
the 1kHz rate is a theoretical maximum. It is not noted there how big one packet is. So doing your calculations vice versa: if you have a packet that is 7 bytes at max, you can transmit up to 1000 of them per second.
The 1kHz mainly refer to the update rate of Low-Level to High-Level processor. This is done via SPI with a much higher datarate. Therefore it also does not make sense to transmit data via XBee more than 1000 timers per second, as it does not change that fast. So that is basically where the 1kHz come from.