They could have changed their mechanics of their interest calculation to be consistent.
So:
28th Feb = They give 0.05% Base Interest for all clients
28th Feb = They give 3.75% Bonus interest for all client (based on INCREMENTAL ADB approach)
20th Mar = They give the dued Exclusive Bonus Interest for Exclusive Clients (based on full balance approach - what was already paid out on 28th Feb).
This is my guess so I will not contact my RM until I see what happens on 20th March.
As long as Item 2 + Item 3 = 3.75% p.a. on my entire Feb balance, that's fine.
So in the end, the numbers are all correct for the 3.8% Exclusive Esaver promo.
I got:
28th Feb = 0.05% p.a. interest
28th Feb = 0.2% p.a. Bonus Interest
16th Mar = 3.55% p.a. Exclusive Bonus Interest
Total is 3.8% p.a. on ENTIRE FEBRUARY 2023 balance. Incremental Balance does not play a part at all.
From what I see, I think SCB made a fundamental change to the way they credit the interest in order to cater for the Normal and Exclusive Esaver clients.
Month End, SCB will give the base interest and 3.8% p.a. Bonus interest using the 'usual' incremental ADB method. So the Bonus interest will not make sense for Exclusive Esaver clients.
Then before 20th Mar (16th Mar in this case), SCB will calculate / make up the difference in the Exclusive Bonus interest such that Exclusive Bonus Esaver clients will get the full 3.8% p.a. interest on the ENTIRE FEBRUARY 2023 balance (Base+Bonus+Exclusive).
I will just use an Excel to keep track and as long I add up the lines and it gives me 3.8% p.a. total interest on ENTIRE balance, that's fine for me.