Quite often, the discrepancy is caused by not including dividends.
And when asked for YTD, some count from the closing price of the 1st trading day of the year. I prefer to use the price on 12/31 of the last year. The gain/loss of the 1st trading day of the year belongs to the current year, agree?
For example, consider VUSUX:
Vanguard shows -18.62% on the fund page,but -18.91% in the summary
Google shows -20.99% under YTD, but -17.87% in the YTD graph
Portfolio Visualizer shows -18.91%...
On 12/31, Yahoo Finance shows the price as 12.78. Today's 8/10 close price is 10.19.
YTD. How about dividends?
There have been 7 dividend payouts worth 0.128. Now, here's a complication. If the dividend is reinvested, you will have less than 0.128 at this point because the fund has been dropping mostly. You buy low, and it goes lower. To be accurate, the price at each of the 7 dividend reinvests has to be accounted for.
Too much work, so let's see what happens if we take it in cash. Our total value is then 10.19+0.128 = 10.318. Then, the YTD return is -19.26%
Hmmm... Does not come close to any of the above numbers.