The reason why you are getting mixed results with AirPlay sync is because you are using 3rd-party software that does not implement the audio synchronization portion of the AirPlay (formerly branded as AirTunes) protocol correctly or even at all.
All 3rd-party non-Apple licensed AirPlay speaker/receiver implementations (i.e. AirBubble, ShairPort, Airfoil, etc.) are created based from reverse-engineering efforts of the RAOP protocol
, and some don't implement the synchronization portion at all. In fact, AirBubble's product description page here
even says that it won't stay in sync with other AirPlay devices. The fact that it works for you is probably luck. I'm willing to bet that if you go just out of reach of your Wi-Fi network with your Android device and cause audio to stutter and then come back into range, it will be out of sync with iTunes and won't correct itself unless you restart the song.
Ultimately, the better the reverse engineering and understanding of the protocol, the better the 3rd-party speaker/receiver will stay in sync with iTunes and other official licensed AirPlay devices (such as your Pioneer receiver).
A comprehensive list of official
Apple licensed AirPlay devices (all hardware) can be found here
. These should most definitely stay in sync with iTunes.
As far as 3rd-Party implementations are concerned, Rogue Amoeba's Airfoil Speakers is definitely among the best. I've seen very few complaints from people online saying they get out of sync with iTunes or other AirPlay devices when using Airfoil.