Example:
You have channel A set at 1V/div, channel B at 100mV/div. You set the trigger level to one division above zero for channel B (e.g. 100mV). Then you switch trigger source to channel A: the trigger channel is changed, but the trigger level is still the old one (e.g. 100mV) rather than becoming 1V now (because 1div = 1V in channel A).