This visualization is by design. The rows are not indented according to the group headers, but instead they are aligned at the most left part of the area. That is mostly because according to the DataGrid's design the group expanders doesn't take extra space (extra empty columns) like in RadGridView for WPF for example, but instead they are drawn under the first column. Even if you manage to offset the rows, you may end with cells misaligned with their column headers.
Currently, there is no mechanism to change this behavior.