Well I've attempted to chuck a few reasons in. He's got less pace and aggression than Steyn, he is often guilty of bowling back of a length and he has often failed to back up 1 good performance with another in the next game. The fact that now he's finding that consistency he's had to bowl on some very flat pitches doesn't help his record, he bowled fantastically in West Indies for example, but still averaged 38 with the ball. It's very hard to explain.
Intriguing topic though. Steyn seems to overperform whereas Anderson seems to underperform. The issue of length is of importance, as the full ball which doesn't swing can still be a potent threat if it wobbles ever so slightly. I think Anderson and other English bowlers are far too quick to 'assess' conditions and bring the length back. I think it comes down to the natural approach too, Anderson tends to jerk the head down slightly moreso than Steyn and I think that makes it that extra bit harder for him to bowl fuller. Although Anderson cannot be faulted for this and it likely gives him extra pace, he tends to deliver the ball from a higher position as he doesn't collapse on the front leg as Steyn does and this can mean that the approach is more 'up and down' than 'flat and full', if I may coin two terms.
I think approach is critical too. Steyn keeps it far simpler than Anderson and perhaps this is due to not having the conventional inswinger at his disposal. I have seen Anderson throw away many a dazed batsman due to throwing in a wayward inswinger down the leg side.
I have also thought that line is critical too. Steyn bowls far staighter than Anderson and again, this may be something that comes naturally to Steyn through playing in far quicker paced conditions than Anderson. The outswing from the stumps approach is far more potent than the outswing from outside off stump approach and also more expensive.
Between us, we have come up with a pretty all encompassing analysis - but again, mostly all quite simple things and yet, it all feels a bit empty. Is
this all there is between a world class sharp outswing bowler and an occasionally very good one?