When medicines are dosed, one of the factors considered is how much of the medicines the body can absorb before the rest is excreted as waste.
In a septic tank, the antibiotics aren't going anywhere and can pose as more than the required quantity to kill off the microbes, even in large containers.
In a septic tank, they flow out with the rest of the effluent into the drainage system. I would think an equilibrium concentration would eventually be reached.