The short answer is that it doesn't. Games for Xbox (or any console) are optimized specifically for that hardware. Therefore, the game will play well even though specs are significantly lower.
Also, consoles typically run games at lower resolutions and at a steady 30 FPS.
It also depends on the specs of the laptop. Some $800 dollar laptops can be fine for games like Skyrim, while others wouldn't play it well at all. It mostly depends on the graphics setup of the particular system.
As for building a gaming desktop, how much are you looking to spend?