Well there can be several reasons for it. Not all currents flow in one direction then you have to take into consideration how high are they riding as they may have discharged ballest before entering port so are subject to strong winds. Could be under power before coming in. That is a few reasons for it and there will be many more. If I am wrong then I had better ask Jim if there is any room under his rock for me to hide.
That's the way the mop flops.
My thanks to Brian for this site.