Why are the secondary market rates displayed on the portal not exactly 90, 180 or 365 days for treasury bills?
The secondary market rates displayed on the portal are for T-bills and bonds are based on the bills and bonds that CAL has in its portfolio. If days have passed since CAL purchased the bills and bonds to its portfolio, that will reflect in the bills and bond rates which are available for sale.
For example, if CAL had purchased a 90-day treasury bill to its portfolio a week ago, today the remaining tenor of the bill reduces to 83 days.
The rates quoted are annual rates. Hence interest will be proportioned for the tenor of the respective bill/bond.