Usually the people that step left when you announce passing are noobs to walking the MUP or they are out for one of their bi-annual walks. One in the spring and one in the fall. Regulars on the trail generally get the picture after a few experiences.

What I'm still trying to figure out is why so many runners look over their right shoulder when I say I'm passing them on the left.

The bells that sound like the more traditional brass bicycle bells are more pleasant, noticeable and readily recognized as being a bike coming up on me while I'm walking the MUP with my wife. Electronic bells are weird and I have trouble recognizing them much less am I able to tell what direction they are from me.