Passive mobs spawn despite /gamerule doMobSpawning false

Not entirely sure what is causing it, or if it consistently occurs (my testing was inconclusive), but on a default superflat world, I had typed "/gamerule doMobSpawning false" , and the game confirmed that it had been successful. I then typed "/kill @e[type=!Player]" a few times to remove any entities and any potential drops. Later, I was confused to find a bunch of villagers in the world, that had apparently spawned in a village just at the outskirts of my render distance. I made sure the gamerule was set to forbid spawning, killed all non-player entities again, and went about my business. As I was continuing to experiment with teleporting entities, I was shocked to accidentally teleport about 20 villagers to myself, again! I hadn't moved to a different area of the map, so the only village near me was the one from before. I again checked the game rule, killed the villagers, and continued, and when it happened again, I decided it was probably something to report. My villagers just won't quit spawning!