Can you show an example? Brushing can't be darker itself but the texture can be more or less course and this might effect the way light reflects off of it but woud be very hard to see. Older gens may also get polished over their life, Its more likely that its the effect of the brushing grain that you can see and it will alsow depend on what model rep you are referring to vs what gen. ITs a bit of a sweeping statemnet as both (unless dirt cheapshitters) use 904L stainless or at least 316l and there is little difference to the eye on either but more the properties of the grade against corrosion.
Different factories use different tools and others use bracelets made by others.