This is speculation, due to me not knowing a lot about the way that halo was programed. But in theory different manufactures do not make there boards (graphics cards) handle the a particular action the same way, but they both yeild the same result. They way these boards handle different functions is how "fast" the board runs a particular game or app. This is why ATI and Nvidia go back in forth in different test. One board does a benchmark better than the other, this means the board is more efficient in that particular area. You may or not have known this, but this is where I believe it ties in.
Halo is a port of the Xbox version. In a port they take the game and "ducttape" it together to work in another environment that it wasn't originally intended for. A graphics card and a processor has to work harder to convert and play a port than it would if the code was intended for it. Thats why you have directx9 boards directx8 boards etc. I said all that to say this. The xbox is basically a computer. It has a nvidia graphics board, intel proc, ram. It seems that it plays better on your laptop due to it being a closer hardware configuration to the Xbox, therefore it has to work less to convert the code making the code run faster. I don't have evidence to prove it but from a programmer stand point it is logical. Thats why ports are always worse than the orginial. Thats because it takes total reprogramming to go from one system to another, unless you "port" it. Thats my opinion take it with a grain of salt. Hope this helps.