Firstly CIMB Fastsaver increase in interest rate is wef 1 Sep.
Secondly, BOC Supersaver interest is calculated based on average monthly balance.
Account balance refers to the Average Effective Balance, which is calculated on a monthly basis as the summation of each day-end Effective Balance for a month divided by the number of calendar days in that month.
If you withdraw on 1 Sep, you get the interest calculated on 75k.
If you withdraw example on 29 Aug, your monthly average balance used to compute interest would be 75k x 28 days divide by 31 days = $67,741
https://www.bankofchina.com/sg/pbservice/pb1/202007/t20200731_18246928.html