Why does my 401k "After-tax contribution" balance vary?

I have a 401K administered by Vanguard. I have not been employed by the company whose 401K this is since July, 2020. Hence, I have not made any contributions to the 401K since then.

Vanguard allows you to view your 401K balance by “Source of Contribution.” One of the several categories of “sources” is “After-tax contributions.” For a while, during my employment, I did contribute some money to my 401K on an after-tax basis.

What I don’t understand is why the amount of money in this “After-tax contributions” varies with time. It seems like this amount should be the amount of money that was taken out of my after-tax paycheck, which of course, is a specific amount of money that hasn’t changed in several years.

However, when I look at the balance in the “After-tax contributions” category for different dates (over the past couple of years), Vanguard shows varying balances (the balances go up and down and seem to more-or-less track with the general stock market movement).

Can anyone explain what I am missing here? I was going to call Vanguard and ask them, but since it’s tax season, I figure they are pretty swamped right now with tax-related inquiries.



It’s the value of the shares purchased with those after-tax contributions. It should vary every day the market is open.

Ah, that makes sense I suppose, but that is not how I have been thinking of it.

Thanks a lot.