This code

#include <stdio.h>

int main(void){
  (void)printf("%.0a\n", 1.);
  (void)printf("%.0a\n", 2.);
  (void)printf("%.0a\n", 4.);
  (void)printf("%.0a\n", 8.);
  (void)printf("%.0a\n", 16.);
  (void)printf("%.0a\n", 3.);
  (void)printf("%.0a\n", 6.);
  (void)printf("%.0a\n", 12.);
  (void)printf("%.0a\n", 23.);
  (void)printf("%.0a\n", 24.);
  (void)printf("%.0a\n", 25.);
  return 0;
}

shows that printf uses both 1 bit and 2 bits for the integer part before the 'p'
(which seems inconsistent).  My guess is it depends upon rounding up.