Does anybody understand why /usr/bin/sed doesn't match gsed's behavior here?

% echo '  LLVM version 13.0.0' | gsed -n -e 's/.*version \(\([0-9]\+\.\)\+[0-9]\+\).*/\1/gp'
13.0.0
% echo '  LLVM version 13.0.0' | sed -n -e 's/.*version \(\([0-9]\+\.\)\+[0-9]\+\).*/\1/gp'; echo

%