By design:
This is expected. The playback time is always relative to the next frame start as this is where all sources played within a frame are scheduled to start (also those not started using PlayScheduled). The reason you're seeing 500 samples is probably because this is the default processing block size used on iOS (actually 512 samples), so that defines the maximum delay. Obviously the lower the sample rate gets, the more likely this jitter is to happen, which is probably why you are getting the best results when running in the editor

To reproduce:
1. Create new project
2. Attach AudioTest.js to camera
3. Change audio settings>DSPbuffer size to best latency
4. Build and run on iOS