Different brands may vary, ...

Different brands may vary, but there are likely several possible reasons. Some may read to the 10th of a gallon locally for leak detection reason a, but only transmit whole gallons. There could also be a lag between the reading being stored in the memory, to being available to transmit as a reading. With wired encoder meters (USA), the transmitter may only interrogate the register periodically, Sometimes  from once a minute to only once per hour or two to preserve battery life.