Because Barret M98B has 100 ms zoom-in time and DSR has 150 ms.
This 50 ms doesn't seem like much, but in reality it's a huge difference, because on top of that 50 ms comes your latency/ping.
So it's 50ms + your ping. The server has to acknowledge the fact that you've zoomed-in before you shoot. And if you shoot before that time - your accuracy will be random.
Now that you know it - you will better understand what kind of bullshit people are telling here, on the reddit forums about quickscoping (for instance - that you can efficiently with Orsis, that has 140 ms of zoom-in time). Don't ever listen to these idiots.

When it comes to sniper rifles - zoom in time is more important than anything else.