From the sounds of it you are referring to the 802.15.4 XBee modules. In which case this is due to a divisor and clock rate issue that you are seeing. As you go higher in rates, the farther out it will become. The only solution for this is one of three:
1 set your baud rate on your device to 112kbps to match the radio.
2 Interface with 2 stop bits instead of 1
3 Lower the baud rate to 57,600bps or lower.