The RadGrid control assigns the "grid" role to a <div> surrounding the <table> containing the grid data (see screenshots attached).
I believe this is an unfortunate assignment as it seems to confuse JAWS because the <table> HTML element by definition comes with an intrinsic role of "grid". So Jaws recognizes two nested grids when focusing the table using the JAWS virtual cursor "T" key (see screenshots attached).
I suggest to assign the "grid" role to the RadGrid's master table and all other tables - or to omit all the "grid", "row" "gridcell" etc. roles, as the table model automatically comes with these roles.