Mob Spawning dont work like it used to in SMP

Mobs stop spawning after some time on a multiplayer server.

How to replicate: Create a world. Build a Mob farm (without spawner). Test it Offline (Singleplayer) and on a server and see how mobs stop spawning after some time in Multiplayer.

I copied my world from my server and tested it Offline . On Multiplayer (only me online) i get like 20 Gunpowder in like 20-30 min. Offline i get more than a stack in like 15 minutes.