Minor Description
The hdfs checksum command computes the checksum in a distributed manner, which would take into account the block size. In other words, the block size determines how the file will be broken up.
Therefore it can happen that the checksum command produces different outputs for the exact same file only differing in the block size: checksum(fileABlock1) + checksum(fileABlock2) != checksum(fileABlock1 + fileABlock2)
I suggest to add an option to the hdfs dfs -checksum command which would displays the block size along with the output, and that could also be helpful in some other cases where this piece of information is needed.