the only time were there are strong variation of link balance is if there is too little traffic in the TRX. In that case you might see “crazy” values.
typically, you need at least 0.1 Erlang @ BH to see “reliable” results. The more the better.
Apart from this case, I agree with you : the link balance should not change too much over time, since the only cause of link unbalance is faulty hardware.