When I use a different set of options with "od", there is no problem:

$ echo ab | od -cx  # your method: 2-byte units in hex
0000000    a   b  \n                                                  
+  
             6162    0a00                                             
+  
0000003

$ echo ab | od -c -t xC  # my method: char-sized units in hex
0000000    a   b  \n                                                  
+  
           61  62  0a                                                 
+   
0000003
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