While reading the part about I2C bus of reference manual of STM8L MCUs, I found that there is this register named Clock control register high (I2C_CCRH). Its 6-th bit, referred as "DUTY", can be used to set the duty cycle under fast mode. When it is set to 0, it means that the ratio of t_low (low phase time bus clock) to t_high (high phase time of bus clock) being 2/1. When it is set to 1, it means the ratio being 16/9.
My question is: What is so magical about the ratio 16/9? In other words, how was this number determined in the first place?