TPS65381-Q1: About WD Open Window time

Hi,

Please tell me about the minimum value of WD's Open Window (WD_WIN1_CFG) time.
The following formula is described in the data sheet.

What is the situation that becomes "-1"?
The actual value of some samples was not ”-1”.
Could you tell me the meaning of -1 in the formula.

tWIN1_MIN = [(RT[6:0] – 1) × 0.55 × 0.95] ms 

Best Regards,
Yusuke