I would assume it would be a valid, otherwise they would not list it on there. Since everything these days is automated it should be fairly easy for their systems to calculate based on the the prices of the funds on the day your contributions are deposited and the value of those contributions. I'm sure there is probably a day lag or something like that.

It all depends on the EXPENSES YOU PAY in your 401K. Those expenses are not apparent unless you do a lot of digging on most 401K's. So you don't really know what they are since they don't make it easy to find. So those returns may be higher in some one Else's 401K given the same funds--depending upon how well your company purchased those funds. (expenses charged)
Most of your funds can be found in your local paper and you can compare those results to your results.