Companies tell you that one factor is that the prices listed in their 401k statement are for units rather than for mutual fund shares. They say they need some cash to deal with situations such as people selling shares. Plus dividends and distributions get incorporated over time into the units and may not precisely match share purchases. Hence the unit is a catch all that accumulates total value rather than share price.

But differences over time should be small. If you compare per cent change in share price with unit value for a given time period, difference should be small.