different growth rate in raised bed

This year I have tried growing some vegetables in a raised bed off the ground about 2 feet. The seeds were set in a north-south orientation. I find that the growth rate in the centre of the rows is much greater, with the growth rate from the centre of each row tapering gradually down to the edge of each row. Looking at the bed side on, the young plants are smallest at the south side , gradually increasing in size to a maximum at the centre and reducing to the same small size at the other end of each row. Can you give any explanation for this apparent phenomenon. The bed is made from wood.