from PC/getpathp.c:538


    if (pythonhome != NULL) {
        char *p;
        bufsz = 1;
        for (p = PYTHONPATH; *p; p++) {
            if (*p == DELIM)
                bufsz++; /* number of DELIM plus one */
        }
        bufsz *= strlen(pythonhome);
    }

The second last line should probably read

        bufsz += strlen(pythonhome);

Hmm, not sure.
Try with 
   set PYTHONPATH=.\a;.\b
sys.path will contains the entries:
   <current_dir>\a
   <current_dir>\b
a multiplication seems necessary.

Before the multiplication, "bufsz" is not aptly named: as your snippet shows it's counting the number of delimiters in the PYTHONPATH.