Does Brokerage Adjust Cost of Your Notes?

[dualstow]

First of all, sorry for the on-topic post. I'll try to get back to preppers, religion, cops, immigrants and vitamins as soon as is humanly possible.

I hold a lot of 3-year notes and other short-term treasury notes for much of my cash portion.
I keep a spreadsheet in google drive, and I pull cost data from the WSJ datasheet which is mentioned in this forum.
What I didn't notice until recently is that the cost of my notes keeps changing.

I am used to Vanguard updating cost basis on individual stock shares, because sometimes the company has some corp action like "return of capital" or something. It's usually not a major change. So cost basis often does not exactly match transaction history, even though they're both coming from Vanguard.

Soon after I buy treasury notes, there may be some change due to accrued interest or something. I don't know. I usually buy notes at auction, by the way, and not on the secondary market.

Only recently did I notice that notes I've held for a long time are still showing different costs from one month to another. Is anyone else experiencing this? I never bothered to look before. I guess it's not a big deal as I tend to hold these instruments to maturity unless I really need the money.

[MachineGhost]

It sounds like they're not all not earning interest at the same time?

[dualstow]

It sounds like they're not all not earning interest at the same time?

I mean, each note or bond has a very specific schedule, which is easy to look up. They all pay twice a year, six months between payments.

Looking more closely, the main one that keeps changing cost is a bunch of 7-YR notes, almost the only thing I bought on the secondary market. CUSIP is 912828RY8.