The ones really close together are freshly launched and still being moved into their final position.
However, all satellites are by design placed into a small number of orbits, see [0]. For example, phase 1 consisted of 72 different orbits with 22 sats each, so each sat would be 1/22th of an orbit behind the previous one - and the orbits are shifted in a similar fashion around the earth. This ensures that there is a uniform global coverage, as a new sat will move over an area right when another one moves out.
