I was reading, and it sounds reasonable, that there might be cultural undercurrents. Kim is only a week or so older than Eric Trump, so that Kim might be looking at Donald Trump as a father figure? Thus, more respect than normal?
I have been watching Korean TV.
One thing that comes across is formality and respect for elders.
Not a father figure perhaps but age.
I believe that Kim sees Trump as 100% trustworthy.
President Trump is strongly charismatic say people who have met and dealt with him. No one in North Korea would dare to lord it over Kim, so Trump would be a refreshing experience, like dealing with his grandfather was.