The sensors collect at different sampling rates because the crystals used in the sensor circuitry are not perfectly identical to one another.  These crystals control the measurement frequency.  Thus, each sensor measures at slightly different sampling rates (e.g. 999.99 Hz vs 1000.01 Hz).  We use time syncing to help with these small differences.  This is also why there will not be an equal number of data points in each log per sensor.