In both cases FIRECalc results say:
"Your plan is to spend $32,000 a year, or 4.27% of your starting
This is correct in that you start with a $750,000 portfolio in both runs. In the SS version you don't start withdrawing until four years after your "starting portfolio", which has no impact on the initial withdrawal percentage. It does decrease your withdrawal rate at the four year point and the result is an improved success rate.
I show a 100% success rate for the run with SS and a 91.5% success rate for the run without SS. In terms of the 4.27% SWR rule of thumb, I'd say those numbers aren't outside what you would expect them to be.
The devil is in the details...
EDIT: Of course the information Dory posted 6 seconds after I hit the enter key might be more accurate than my infomation. He did write the program after all.
But I stand by my "devil is in the details" comment. 8)