OK, I'll buy that. It's consistent with what I've observed. I'm guessing that comes from the game code?
If I'm interpreting what you are saying correctly, if a CS settler spawns 5 hexes from me, and I move 1 hex towards him and settle on turn 1, the same thing would happen?
Still, it makes little sense for the game code to allow for spawning on a mountain (although it does explain the 1 or 2 occasions where I only met 15 CS).
My airplane crash theory shouldn't be completely ruled out, especially in 4000 BC, right?
The Lua apprently placing it on a mountain to begin with is the biggest mystery. You could play 100 more games and probably won't see it happen again.
Some of the city state kills are turn 0 when the LUA gave up trying to place the last city state's spot, this is much more common than the turn 5/6 kill. Way back in the days of Vanilla you'd got notifications if an unknown city state was killed by any means.
The most common single reason for there to only be 15 city states to meet in a game setting that called for 16 though (in a game that doesn't include Austria / Venice) is that some AI conquered one of them before you met.