Perhaps the 738 is similar to the 744. Sometimes the valves remain slightly open after engine shutdown (The OFF light doesn't illuminate in the overhead switch). When you switch off the bleeds (with the overhead switches), 99 times out of 100, the valve will close completely. This seems to happen irrespective of bleed duct pressure. The electrical close command and the springs in the valve must give the valve that extra bit of incentive to close(?). Or perhaps there is enough air pressure trapped somewhere in the system to assist the complete closure when the electrical command is given.
Regards
NSEU