It is just display error. Opponent loses some units in reality despite the fact that you see 100% heal for both sides.
This is not true. You can request duels while moving. You can be challenged while recalling and end up in friendly territory. It's based on your location when "accept" is pressed, and people manipulate this all the time. Not sure what your motives are for lying.
This was the most perplexing one for me: https://www.youtube.com/watch?v=jPJMkZSm60E