Laptops with combo Intel + Nvidia GPUs (Optimus) are set up so the Intel GPU always drives the screen. The Nvidia GPU acts as a co-processor. The game renders a frame on the Nvidia GPU. The completed frame is then sent to the Intel GPU, which displays it. It's basically like vsync is always on, with the two GPUs acting as the two vsync framebuffers. This is why you can select in software whether a game will use the Intel or Nvidia GPU. If you select the Nvidia GPU, it does the above double buffering thing so the Intel GPU can display the completed frame. If you select the Intel GPU, the game renders on the Intel GPU which also displays the completed frame.
A few gaming laptops are set up in this way only for the laptop display. The external display hardware is hooked up directly to the Nvidia GPU, so you get a more "pure" (desktop-like) gaming experience when connected to an external monitor. I don't have a definitive list of which laptops are set up this way. But if your framerate is higher with an external monitor, your laptop is likely one of them. Battery life suffers when using an external display, but presumably that's not a problem since you can plug into AC power.
(A few older or poorly coded games don't work with Optimus. They're programmed assuming a computer only ever has one GPU. So they detect the Intel GPU first (since it's driving the screen), and never get around to detecting the Nvidia GPU. So these games will only run on the Intel GPU on an Optimus laptop. But if your laptop has the Nvidia GPU connected directly to the external display, you can get these games to use the Nvidia GPU by playing the game on the external monitor.)