In the game, we see that one can travel from time period to time period with time either freezing from the point one leaves the time period (1999 apocolyspe) or flowing at a constant amount like in real life. The reason, I guess, Robo is still there because remains in the same spot over several hundred years, versus the characters who are either jumping instantaiously from the moment they left the time period to the moment they arive or time actually passes between when they leave to when they return.
Imagine two people in a room, one leaves the room and enters the same room except now time has passed. He takes the person from this new room (which is the same as the old room, except it is a diffrent time period) and takes them back to the "first room (the past)", and of course, the secound person is still there (which he has to be in order for the older version of himself to exist) and in theory, the secound person will have full memory prior to the event, except it would have been from his younger state.
Perspective of first person (Chrono and friends): Leaves secound person behind, meets him in future, brings him back to the present, now there is two versions of the secound person, the past and future.
Perspective of secound person (Robo): First person leaves and either a porportional amount of time passes (the amount of time the first person is in the future) or he instantaniously reappears with secound persons future self. Supposedly the future version leaves (otherwise we get an infinitly increasing paradox, much like how the travelers leave). He then waits until the first person arrives (from the past) and takes him to meet his past self.
Because none of the other characters are left in one time period for more than a few days, they cannot transend the great distances between the period like robp did, thus they cannot meet themselves!
