YTD return is not the same as annualized YTD. For example, if I make 3% from Jan thru June, then YTD return is 3%, but annualized YTD return is 6%.
I think with XIRR() one has to put 12/31/2011 as the last date as well, otherwise you get the wrong YTD as well.
Last week, both bonds and equities had returned 3% to 4%, so it didn't matter much what your asset allocation was: You had about a 3% to 4% return for the most part.
This week, with equities down and bonds up, bond-heavy asset allocations have done much better than equity-heavy allocations.
If one looks at some Vanguard Target Retirement fund YTD returns, then one can see for the stock:bond ratio in those funds what an index fund YTD return would be. For example, Vanguard 2030 has a reported 18.5% bond allocation and YTD of 1.34% while Vanguard 2010 has a reported 52.2% fixed income allocation and YTD of 3.59% (thru August 2).
That Vanguard 2030 YTD return of 1.34% would be about 1.34% * 12/7 = 2.3% annualized YTD return.