Need Clarification - Lag example in PMBOK Fifth edition

Hello 
On PMBOK Fifth Edition Page 158, there is example of lag.
Activity 1 (Preceding activity) = Write Draft
Activity 2 (Successor activity)= Edit Draft
Activity 1 is connected to Activity 2 by Start to Start with 15 Days Lag
 
Now I am not understanding, why PMBOK has mentioned SS - 15 Days (Lag).
Why PMBOK has not mentioned SS + 15 Days (Lag)
 
Please explain this.
Thanks
 

Lag is represented by negative sign....
 
Refer Rita for clarity....
 

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