The error rate shown on the FSiA6 is only available when the Rx is close to the Tx and does not show up when the bird is flying. It is a measure of the bit error rate between the Tx and the Rx and will rarely show zero, ( I have never seen it reach zero on my transmitters/receivers.
Due to the nature and protocols used in the FSiA6, (a version of Spread Spectrum), as is the norm for most modern radios the bit errors are corrected before the transmitted data is forwarded to the servos etc. Most SS radio systems can function quite reliably with bit error rates of well over 75% without signal corruption or communication problems.
The Tx can support the older AFHDSS protocol and sometimes can start in the wrong mode causing an elevated error count so use the bit error rate as an indication of it being in the correct Tx mode. In the wrong mode the servos can jerk around and not operate smoothly as well as the Tx range is markedly less!
Have fun!