In setting aside the Full Retirement Sum in my Retirement Account, why are my Special Account savings transferred before my Ordinary Account savings?

The savings in your Special Account (SA) are meant for your retirement needs, while your Ordinary Account (OA) savings can be used to meet shorter-term expenditure needs such as housing. Hence, when you turn 55, SA savings are first transferred to your Retirement Account (RA) to help you set aside the retirement sum, which will eventually be streamed out as monthly payouts when you reach your payout eligibility age. 

Did you know? If you wish to earn higher interest on your OA savings (which earns at least 2.5% interest* per annum) and boost your monthly payouts, you can make a CPF transfer to your RA (which earns at least 4% interest* per annum), up to the current Enhanced Retirement Sum. Do note that for CPF transfers to RA, your SA savings (if any) will be transferred first followed by your OA savings.

* Based on the current interest rate floor on Ordinary and Retirement Account monies.

This information provided here is sourced from the CPF website.