The interest rate is dependent on the utilization rate: As the bigger share of BNB deposited into the bank gets lent out to Yield Farmers and Liquidity Providers, the interest rate also goes up. The interest rate model means that there are two distinct sweet spots for utilization: 80-90% (where the interest rate is actually fixed) and 90% for highly active market times (with a steep rise to disincentivize 200% utilization).