mobgis

I'm new to remote sensing and am trying to get my head around it all so apologies in advance for the noob questions. I've been playing around on envi and have produced some NDVI and LAI images- they look exactly the same but the value ranges are different what is the reason for this? Furthermore is anyone able to explain their relationship to me in easy to understand terms? (eg when LAI increases why does NDVI increase? Thanks