I don't understand this, but it happened.

I'm doing an animation, 300+ frames, (not that it matters how many).

I had only a base layer, which I was happy with, stuffed as it was by toil with sweet keyframes.

All that was left was to do some arm movements, which were totally absent, (the arms were static so far).  So, I create a new layer and set it to add (instead of replace).  Well, I dabbled a few minutes then reached a point where I decided to delete the layer and start over. Well, after I deleted my new layer, my figure's arms were in zero'd position, not where they were before, in a neutral standing position (hanging at the sides). 

That means, deleting an animation layer affected the keyframes of the layer below!
WTH!
huh? Am I insane, or is THAT insane? Just exactly how does this cobbled-together legacy village do it's adding and replacing? What should I do next time?