I think it is just player priorities.
For example, I play
and my priority is farming. I have a tendency to ignore trades and just farm under tower early.
My opponents frequently harass me but miss CS in doing so. I usually end of with similar to higher CS then them, most of the time in the first 15 minutes and then I begin to pull away.
Players appear to prioritize trading and early kills rather than just farming. This is definitely the case in Silver, but a lot of Gold players do this as well.
Maybe it is just from my perspective as a Nasus main, but I rarely play in a lane where my opponent and I trade CS. I actually think that is more effective against Nasus than trying to harass him, since typically my opponent can exert more map pressure early than I can.
If you watch Sirhcez's game (a diamond Nasus), he frequently receives way less harassment in laning than I do, because the players prioritize CS.