please refererence to http://www-03.ibm.com/able/guidelines/web/webtableheaders.html.
see example 2.

complex data tables are defined as having two or more levels of row or column headings or headings that span multiple rows or columns. complex data tables require additional markup to explicitly associate the heading with the data. in the data cells, the headers attribute is used on the td element to specify which heading cell, via the id attribute on the th element, is associated with a specific data cell.
test build:2.6.0.v20100301
by setting bookmark in xtab cells can resolve this issue.