I agree w/the bell, but I think, depending on the scenario, the 'on your left' makes some sense.
I regularly walk in the valleys, along the multi-use trails. Walkers in particular, but sometimes cyclists too don't stick to the right, they wander to the middle or even left of the trail.
I can see where a cyclist approaching from behind, at speed, wants to make sure than the warning doesn't send the person in front to the left, and into their path.
Calling out 'on your left' in that case is a clear direction to stay where you are, so you can be passed.
I get where that may seem self-evident. just with a bell.
But some people hear a bell and think its a 'get out of the way'; when in fact the cyclist is already willing and able to go around, they just want predictable movement.
Some people are not keen followers of pedestrian/cyclist etiquette.
Does it make the same sense in a uni-directional bike lane, where both riders appear to be experienced? Probably not.