I think the ping is showing relative to you, hence why yours is 0 or "-". Just like F1 when the leader's lap time is shown as "interval" and everyone else a split of that.
It's a poor implementation in an online game, but then so long as you aren't lagging out then I can't say I care tbh.
Nope, probably just the usual Dice bugs. Two other players had - for ping, in a game I played yesterday. And everyone will tell you the same ping time when asked, so that is your theory disproven.