I fitness walk as well as cycle.
Whenever I'm walking and I hear a cyclist coming up behind me and calling out "On your left!"- all I usually hear is "(mumble mumble) LEFT!!" And I have very good hearing. I never had a clue what "On your left" meant until I started cycling. To me it's no wonder at all that pedestrians and walkers tend to obediently jump left when they hear "(mumble mumble) LEFT!"

I myself prefer to call out loudly and cheerfully ahead of time "Coming up behind you!" or just even "Good Morning!"- and I ring my bike bell too. The people ahead, no matter what their method of locomotion, always then look back at me and can see I'm approaching on their left side to pass. It's always good not to startle elderly walkers, too. What peeves me is the people with their earbuds in who don't hear a THING no matter how loudly I call out as I'm approaching. They get scared and jump as I pass them. Oh well.