Since, we are integrating over the hemisphere we get the <= 1 condition. This could be because some of the light gets absorbed (or undergoes subsurface scattering? Are they the same?). If we were to integrate over a sphere that might integrate to 1?
@sickgraph Really not about sphere vs. hemisphere; as you say, if the material absorbs any light at all, the integral will be less than 1. This is true whether we're just reflecting (hemisphere) or reflecting and transmitting (sphere).