W401k is the expert who can probably explain better than anyone else. Perhaps he will join in soon. Meanwhile . . .Previous posters have noted that 401k administrators often need a bit of flexibility to deal with things like dividend distributions, redemptions, and cash not yet invested. Hence, they usually use units to divide the total assets of an investment between participants.In the case of a publicly traded mutual funds, you should be able to compare total return at the end of the year for the two and find them similar. It's bookkeeping details that make the numbers a bit different.