D/ST is one of the most predictable fantasy positions. However, that only means that other positions are less predictable. The randomness in fantasy can be overwhelming, and it’s not much different for D/ST. So here's how I think you should look at it:
Control: The key point is that it's >60% about the opponent.
Attitude: Don't set your expectation for those surprise booms. Look at higher ranked D/STs as a chance to limit the downside. Especially since D/STs can score negative unexpectedly.
Numbers: Expect 8-ish points on average over the season, and consider 3 points as a reasonable “floor” from a week’s top-ranked D/ST. Why is that reasonable? Firstly, the average D/ST score (ESPN default) is around a low 5 points. Secondly, typical error is around +/-5ish points, which means a 5 point loss is normal enough to consider “unpreventable”. The point is, it doesn't make much sense to get upset or regret a D/ST result of 5-6 points. Yes, double digits might happen, but your season average should still end up 8, meaning lower scoring weeks have to happen. It's always frustrating to see other scores blow up (which happens every week), but try to feel glad when you've hedged against a negative.