One important detail about BER (bit error ratio) is that it is an average value. If you use it as such there is no problem. But for if you have link whose BER=10^-4 and you transmit 10000 bits over that channel it is likely you won’t get any error at all. The reason is that BER is an statistic but not a RULE.
If you want to learn more abot what this means, read on this interesting but yet accessible document.