TPS92692-Q1: Supply input, battery crank voltage: why isn't it as same as VIN(6.5V)?

Part Number:TPS92692-Q1

hello

For  the character VIN, crank descriped in 7.3 on page 5,   why isn't it as same as VIN(6.5V)? 

If I use the DC Adpater as power input,  what is the minimum value of VIN for TPS92692?  How about the battery source ?

why they have different minimum value of VIN?

Thanks