You are right. Roughness in Principled BSDF which has its glossy component set to Multiscatter GGX(default) will look different than roughness you would get by simulating tiny surface imperfections on a glossy shader set to 0 roughness(in your case it’s not 0, so you get some mix of Multiscatter GGX on top). I’ve done tests for diffusion shaders how tiny surface imperfections change the look of a surface. That’s for a diffuse shader though and with actual geometry, not bump maps. You know why I haven’t tried mirror surface with imperfections? Because I cannot render them. It takes too long. I have a feeling if you control surface roughness with a bump map, it’s very inefficient as well and it’s probably not the best way to atchieve good results, because actual roughness algorithms are way better for this. And you are doing this if you use bump map too small to actually be visible to change how surface reflects the light. But, sure, it looks different. It’s just not a good kind of different. You get results that are less plausible, the render time increases significantly or it gets cleaned by the denoiser anyway and at the end of the day this is not how you are supposed to do this, what you get may be less realistic and less predictable. That’s just not a very good workflow. I would not recommend this. You might end up with a physically implausible result that will look weird and it might be hard to tell why or in best case scenario that just doesn’t matter and can be done more efficiently. My advice would be to develop the look in ways that are intended to be used for this. I think same look you like can most likely be achieved controlling roughness and other shader properties. Bump maps are not for controlling how the surface as a whole reflects light, they are for simulating bumps. It’s just an advice.