Having looked at the stats and browsing some scorecards, I think the people responsible for the award got it pretty spot on. Sachin was indeed pipped by both Sehwag in tests (who scored more runs at a higher average), and de Villiers in ODIs (who scored more runs at a higher average). Sachin may have been able to take the ODI award out if he didn't rest in so many games I guess, but nonetheless, de Villiers has been pretty damn epic in ODIs.
Sachin:
During the period under consideration for the award - August 24, 2009 to August 10, 2010 - Tendulkar scored 1064 runs at an average of 81.84 in ten Tests. He made 914 runs in 17 ODIs at 65.28, and also scored the format's first double-century against South Africa in Gwalior.
Sehwag:
Sehwag scored 1282 runs in ten Tests at an average of 85.46 between August 2009 and 2010, which was the period under consideration for the award. He was also named in the Test Team of the Year, which was captained by MS Dhoni.
de Villiers:
During the voting period, de Villiers helped his side to ten victories in 16 ODIs, scoring 855 runs at an average of 71.25 and at a rate of 103.38 runs per 100 balls faced. In that time he hit four fifties and four centuries, and also picked up five dismissals in the four matches in which he played as wicketkeeper.