That's this wonderful thing called latency. It's the result of there being a delay between your DAW sending MIDI data over the network to your iPod actually getting it. Because it takes time for the MIDI data to go out from your DAW, over the network, to your router, then from the router to your iPod, the MIDI data isn't always actually synced up to the time in your DAW. I tried to do the same thing with Ableton and several other software synths I have on my iPad. What we really want is some hardware-based solution, in which we plug in our iDevices into a hub that plugs into our computers. The DAW sends MIDI data to the hub which then sends it to the iDevice. Because it's over a physical connection, instead of being routed over WiFi, it's going to have a much lower latency, and be much more in time with your song. You'll want something along the lines of the
iConnectMIDI, if you have some cash to spend and really want to get the most out of Animoog and other iPad synths.