spoke with the 401k company today (actually got the CEO, somehow, because of a power outage in san fran...). he said that because FSKAX and VTSAX track different indices, you end up with different distributions of cap gains and dividends between the two.
the total fund performance (i.e. YTD % return) is all encompassing of fund pricing, gains, and dividends. therefore, comparing YTD % returns between the two is a fair comparison in an account where there are no tax implications. if it's a taxable account, then the type of performance matters more.
so far, YTD:
VTSAX = 17.99%
FSKAX = 18.02%
this is the same spread as the last time i checked, and i think, but am not certain, that the delta is a result of of the delta in the fees (0.004 vs 0.0015)