The Sean
Cricketer Of The Year
To follow up on this, after 21 Tests Kallis was averaging 30.78. His highest average - achieved in his 111th Test - was 58.20. This gives a high/low differential of 89%.As for SJS's question of which great batsman with a very high average has such a gap between his highest and lowest points after 20 Tests, I haven't crunched the numbers yet but I'd suggest possibly Kallis? He took quite a long time to really get going as a Test batsman, but when the runs started flowing his average really exploded.
Even if we take the position that - like Tendulkar - Kallis' volume of Tests means that 20 isn't a big enough sample size, consider this. After 50 Tests, Kallis was averaging 41, which compared to his career high is still a high/low differential of 42%.
There might be players with bigger differentials, but I can't imagine there are many.