Imagine your laptop (excluding the monitor) is split in half hamburger bun style. The top case (literally the top 'bun') includes the battery, trackpad and keyboard. The bottom case includes your logic board and some other components. (sorry was wrong about this, see posts below. Logic board is separate altogether.)
If you needed a trackpad replacement and bottom keys, they likely just replaced the entire top case as the parts all come together. In this case, they replaced your 2016 topcase with a 2017 topcase.