As it happens, I have the same model, and I'm actually using it write this answer. I just tested by completely disabling the Intel card through the BIOS, and running the Experience Index again. The number did not change.
I'm fairly certain that by default, Windows uses the "best" graphics card you have installed. Changing anything in Nvidia settings won't change the fact the OS can still see the Intel card, but regardless, it uses the Nvidia card to benchmark.
This related KB article provides some insight into WEI.
For graphics adapters, both DX9 and DX10 assessments can be run now. In Windows Vista, the tests were specific to DX9. To obtain scores in the 6 or 7 range, a graphics adapter must obtain very good performance scores, the adapter must support DX10, and the driver must be at least a WDDM 1.1 driver. For WDDM 1.0 drivers, only the DX9 assessments will be run. Therefore the overall score is capped at 5.9. For multicore processors, both single-threaded and multithreaded scenarios are run. With levels 6 and 7, we intend to indicate that these systems will be rarely CPU bound for typical use and very suitable for demanding processing tasks and for multitasking. As examples, we expect that many quad-core processors will be able to score in the high 6 to low 7 range, and we expect eight-core systems to be able to approach 7.9. This scoring has accounted for the very latest microprocessors available.