To reproduce:

On RadGridView's ToolTipTextNeeded event use the following code:

void Grid_ToolTipTextNeeded(object sender, ToolTipTextNeededEventArgs e)
{
    e.ToolTipText = "Sample ToolTip Text";
    e.ToolTip.IsBalloon = true;
}

You will see that the Tooltip will be displayed too much below the mouse cursor

Workaround:

Add offset to the position of the ToolTip so that it is displayed at the cursor's position:

void Grid_ToolTipTextNeeded(object sender, ToolTipTextNeededEventArgs e)
{
    e.ToolTipText = "Sample ToolTip Text";
    e.ToolTip.IsBalloon = true;
    e.Offset = new Size(e.Offset.Width, e.Offset.Height -30);
}