Your frame count depends on the sample rate. Your latency is probably 0.012s or 0.011s which explains the frame count of 470/471. I figured this is not always a power of 2, especially if the sample rate is 48000 (as is the default case in newer devices since the iPhone 6s).

However, I highly recommend to set the latency to a value that you always get a frame count that is a power of 2. Otherwise some (badly written) plug-ins won't work correctly.