No, means, if you attack with lets say 13 soldiers.
enemy has 100% defenders => 8 run out to defend, 1 stays. now there are (13-8) 5 soldiers in front of the fortress fighting against the last man. lets say 4 die, then there are again 4 fighting and one is in front of the fortress but if the one in building defending soldier dies, the fortress is lost very fast. But if you have luck and maybe 3 soldiers come back, then there are 6 missing which will be called from a storehouse.
enemy has 50% defenders => 8 run out to defend, 5 stay. now there are (13-4) 9 soldiers in front of the fortress fighting against the last 5 soldiers. lets say 2 die, then there are again 2 vs 2 fighting, and still (9-2) 7 soldiers against 5 in the fortress. Because you send 4 defenders, 4 soldiers will be called to defend the building (inside) therefore now are 6 soldiers vs 5 left in front of the building, but 4 are reoccupying => 6 vs 9 again, now you will not lose the building.
enemy has 0% defenders => 0 run out to defend, 9 stay. now there are 13 vs 9 soldiers in front of the fortress fighting against the 9. you will lose the building because there wont come new soldiers.
The problem was, that you could set 0% defenders and build a storehouse full with generals next to the fortress => was occupyied until you had 0 soldiers. Meant at the end you could just do [yourgenerals]-[enemygenerals] = if - you lose, if + you win.
kind of boring.