It was chosen for future expansion. Kinesis was envisioned to be a much larger-scale Kafka + Storm (storm was the streaming programming framework popular in 2012, it was since falls out of favor).
128bit might be accurate, I meant more along the lines of they are non-contiguous and don't seem to be correlated with the amount of records actually being written to a stream.
It was chosen for future expansion. Kinesis was envisioned to be a much larger-scale Kafka + Storm (storm was the streaming programming framework popular in 2012, it was since falls out of favor).