Of course I understand the level of difficulty impacts the scoring, but how many are you high verbal scorers getting wrong???
Even I had the same question. We cannot get the answer from those who are consistent because they cannot understand our position. This is what I found after some analysis.
1) Concentration plays a big role. If you are fresh, you will get good score.
2) Experimentation. If you try something new, your score tends to swing.
3) There are also swings based on question type. Like I was not good with inference questions. Hence, questions like those tend to affect my score.
Finally, the only strategy is found is to devise a very concrete plan and stick to it (like a machine, no matter what the consequences are). It helps me to get rid of pressure and will make you (at least) feel consistent.
Many people dream, but some wake up and work towards it.