What is the reason why CPF say by topping up your CPF SA in January each year earn more interest? Does that mean other month I will earn less interest?

If you top up Jan, by Dec you would have earn interest from Feb to Dec.

If you top up Dec, and assuming you did not invest in this topup amount to earn some returns between Jan to Dec, then Yes, you would earn more by toping up in Jan.

Top up ewrlier earn interest for longer period

If the objective is to benefit from personal income tax deduction (for next year's assessment) and maximise interest to be earned from the cpf top up, it is certainly worthwhile to do it nearer to the end of Jan.

It depends on your comparison

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if you are comparing between topping up in Jan 2022 vs Dec 2022 (Jan 2022 will accumulate more interest, e.g. topping up earlier in the year is better than topping up later in the year)

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however, if you are comparing between topping up in Dec 2022 vs Jan 2023 (Dec 2022 will accumulate more interest)