Why they differ?
Glossy material (Roughness = 1) is diffuse.
All the glossy material does is provide a reflective component that one would likely mix with other shading nodes, much like how the diffuse material only provides diffusion.
The shading nodes in and of themselves are not meant to be used alone in most cases because they only provide a basic component, a lot of convincing materials would have two or more components which would be mixed with each other using the mix or add shader nodes.
diffuse use lambert or oren nayar model
it not reflect environment
glossy reflect environment using reflection cone (=rougness)
I like Maxwell Render reflection system there is only one parameter: Roughness.
100% - diffuse, 0% - mirror