Synchronicities are something that "appears” in an individual person’s life, usually at some particular stage or around some event. Therefore, we can assume that there are certain patterns in people’s lives when archetypes are triggered, resulting in greater likelyhood of experiencing synchronicities.
Jung’s essay on synchronicities is centered around the notion of causality: he points out certain phenomena that (according to him) cannot be causally explained, and in general he claims that the law of causality is insufficient as a principle for scientific explanation. What does he actually mean by terms such as ‘causal’ and 'causality'?
The notion of projection is a broad umbrella which covers a variety of different ways in which unconscious contents might appear, to a subject, as something they notice in the external world. Presumably, some archetypal structures are best expressed in personified form, whereas others lend themselves to a different, more abstract representation: as relationships, as connections.