Contents of sensor_msgs/Image

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I am subscribing to a topic with message type sensor_msgs/Image. I am expecting the size of data array to be 3686400 (Details of the calculation and the problem in detail can be found here). But the size of the array comes out to be 3690487. I believe the additional elements contain additional information required for the system (padding, parity or some other technical requirement). Is that right or am I missing something?