I think you will find its due to the constraints near the Needlea and Hurst point.
All deep draft vessels will aproach from the east of the island (NAB tower), they will have to take a southerly passage past the Brambles bank, turn to the north on the western side of it, head north easterly and then turn up into southampton water.
The main channel is shown by the blue lines on the pic, the north channel by the magenta ones.
To get live vessel locations on the solent try http://www.ais-live.co.uk/AIS%20Live/aissolent.html (http://www.ais-live.co.uk/AIS%20Live/aissolent.html)