Here, sed is used to prepare object files for comparison via cmp. On my Darwin 15.4.0 machine, LC_CTYPE is set to UTF-8 (by default, I believe). Under these circumstances, anything sed is made to read will be treated as UTF-8, prompting it to signal an error if it is not, like so:
% sed s/a/b/ <(head -n1 /dev/random) >/dev/null; echo $?
sed: RE error: illegal byte sequence
To make sed work as expected, I need to set LC_CTYPE to C:
% env LC_CTYPE=C sed s/a/b/ <(head -n1 /dev/random) >/dev/null; echo $?
Without this change, sed will exit with an error for every single file that it compares between phase 2 and phase 3, thereby making it look as if the differences were far larger than they are.