The first few runs a batsman makes in a Test innings are usually considered to be the most difficult ones. The batsman is still gauging the pace and bounce of the wicket, the movement of the seam, he's getting his eye in. These are the times when a batsman is most vulnerable to being dismissed. This is especially considered to be true of opening batsmen, because they face a brand new cricket ball. They tend to average less than middle order batsmen because of two reasons - they have fewer not outs, and they face the new ball more often.
For example, if you compare Sachin Tendulkar and Michael Atherton, the above claims are all true. Tendulkar (Average 55.44) and Atherton (37.70) are both among the top 25 run scorers in Test history. Tendulkar reaches every score threshold in the below graph more frequently than Atherton. In 88% of his Test innings, Tendulkar reaches at least 5, in 53% of them, he reaches 30. For Atherton, the corresponding figures are 79% and 44%.
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Other factors in such an analysis are technical ability, skill and talent. Tendulkar is a particular good standard to use in this, because he is considered both supremely talented, and supremely technically proficient. It is reasonable to assume that batsmen with better techniques will have an easier time starting their innings.
A few other cases tell a different story. Here are a few more examples. In these examples, a few striking things become apparent. Batsmen who ended up with batting averages 10-12 runs lower than Tendulkar's were just as likely as him to reach 20 or 25. The examples of David Gower (average 44.25) and Graham Gooch (42.58) make this clear. Even Virender Sehwag gets to 30 as often as Sachin Tendulkar, but falls away. Mathew Hayden had less difficulty, if anything, than Tendulkar in getting to 30, but after that, "Matt The Bat" tended miss out more often than the Indian. In Viv Richards' case you can almost see his boredom in the graph after getting to 60.
Jacques Kallis and Ricky Ponting have done better than any other batsman when it comes to matching Tendulkar's run scoring. Rahul Dravid and Brian Lara managed this quite well too, albeit in contrasting styles. The Lara - Tendulkar graph shows a classic comparison between the two.
Among elite batsmen then, perhaps the difference is not technique, or luck (which must even out as per the law of large numbers), but something else - discipline. The difference between being Graham Gooch and being Sachin Tendulkar, is the difference between being very disciplined and very very disciplined.