It is so because we consume many plants as we are herbivores and the insecticide is spread on many plants where as it is less in amount for one plant but in gather is amount increases and since we eat too many plants accumulation of chemical is high in us.
E.g. :- 1 plant = 0.05 ddt (and i man comsume 40 plants )
40 plants = 40 ×0.05 =2 ddt
so man have consume plants having in total 2% ddt.
hope u would get clear now