Paradox of using CPF-OA to buy T-bills

Paradox of using CPF-OA for T-bills & investment.

Usually 1 Jan is a routine activity for CPF Balance stock-take.

1.  Check annual interest for the past year; in this case Whole Year Interest for 2022.

2.  VCMA to BHS from $68,500 to $71,500 by $3,000 cash topup, VCRA to its ERS from $298,200 to $308,700, with $10,500 cash topup.

3.  Subsequently another VC3AC by ~$8,000 cash topup to 3 CPF accounts (OA & SA, since MA reached its BHS). 

This is to max CPF topup to its yearly $37,740 for Seniors whose CPF contribution rate is reduced after age 55 and then after age 60.

4.  Subsequently few cash topups to refund MA withdrawal by Medical premiums; maybe $3,000 for whole year.

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I noticed that my total whole-year interest (for 2022) actually dropped and less than that of 2021.

I expected its interest $33k+ for whole-year 2022.  But it was $30k+.

Reason: I use OA to buy 6-mth T-bills.   T-bills interest earned is returned into OA Balance, not accounted as interest paid by CPFB.

I am writing to CPFB to seek clarification.

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Without intentionally VHR to our OA, I think that one should achieve CPF Balance of $1.5M in age 65, singly.

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Sometimes I wonder why would one topup by VHR for a mere OA 2.5% safe risk-free interest, then withdraw them to invest in T-bill to earn an extra 1.x%;

For me, topping up max RA to ERS, BHS to MA is necessary to enjoy the risk-free 4%.  (Some would say 5%~6% for first $60k in CPF Balance).