Inferrable roles inside grid

When an HTML <table> is declared as role=grid, why do <tr>s have to be  
declared as role=row?

Also, when the table acts as a grid, are all cells gridcells or are  
some not? Why is this distinguished by role instead of tabindex, if  
the salient difference of a gridcell compared to a table cell is  
focusability?

-- 
Henri Sivonen
hsivonen@iki.fi
http://hsivonen.iki.fi/