1201 STX ISSUE

cc120xSpiWriteTxFifo(txBuffer,sizeof(txBuffer));
  trxSpiCmdStrobe(CC120X_STX);

// Wait for interrupt that packet has been sent.
                // (Assumes the GPIO connected to the radioRxTxISR function is
                // set to GPIOx_CFG = 0x06)
                while(packetSemaphore != ISR_ACTION_REQUIRED);
  // Clear semaphore flag
                packetSemaphore = ISR_IDLE;
  HAL_NVIC_EnableIRQ(EXTI4_IRQn);
  cc120xSpiReadReg(CC120X_NUM_TXBYTES, &txBytes, 1);

question: 

1. when i send 1 times, the txBytes is 0x20;

2. when i send 2 times, the txBytes is 0x3f;

3  when i send 3 times, the txBytes is 0x5e;

4. when i send 4 times, the txBytes is 0x7c;

5. when i send 5 times, STX status byte is 0x7f,which denote TX FIFO has over/underflowed.

why the Tx FIFO error?  the txBytes is must 0x20 when i have finished send the packet?