This was the work of CCIP for RUB
All the major convoys have 2 parallel courses joined together so a convoy might go stright on or make a course change, all done by randomness.
Makes for one great convoy chase and a great sense of satisfaction when you find the convoy, especially if it was not where you expected it to be and made a D to go over there instead.
NYGM have expanded on this and created random speeds for each waypoint (as we have done for every ship in the game) so if you had a report of 6 knots at the next waypoint the convoy may go up to 8 knots!