Why do some plants do good at 1:1 ratio in inter-specific competition?

So, I did experiment with marigold:brassica in mixed culture density in order to see inter specific competition effect on marigold.As a result, highest individual biomass of marigold was found at 8:8 ratio while the lowest /constant was at like 4:12 , 16:0. I dont really get it why?|||Good