The timestamp is recorded at the time of arrival, and they will not typically be spaced a sample period apart as the bluetooth data does not always arrive at user level processes in a real-time manner.
The timestamp is useful for relating samples, within a few ms, to when they were received. Bluetooth latency depends on the OS and hardware but is ~15ms for most of our setups.
For numerical analysis / FFTs assume the sampling frequency is regular and the stated frequency. The EEG sampling is happening with great regularity even if the samples are not arriving to our process from bluetooth with similar regularity.
We do calibrate the frequency but it is affected by temperature, and can shift about ±.25% target. The feedback in muse-io as to the receiving frequency is averaged over a few seconds and is probably the most accurate.