With biomagnification, if you're eating the plant you'll get some exposure to said toxin - and it's unlikely you're eating just that plant, but if you eat a her

With biomagnification, if you're eating the plant you'll get some exposure to said toxin - and it's unlikely you're eating just that plant, but if you eat a herbivore that eats that plant - and a herbivore can eat a lot of a specific kind of plant, you'll be exposed to a higher concentration and then if you eat the predator of the herbivore you'll get an even more concentrated dose.