TL431-Q1: Output Voltage Calculation

Hi team,

 I am confused about some content on the datasheet, could you pls help to give me some supports:

1. I want to know the formula: how is -Iref *R1 understand?

2. I found that Figure 21 and Figure 27 are the same, but the formula is different, what is the reason?

Looking forward to your reply, thanks.