[NOOB] Export subkey

[Arthur Rance]

Hello,
I'm a noob and I'm going to export a subkey :
$ gpg --list-keys
pub   2048R/12345678 2010-01-01uid                  Arthur Rance <arthur_rance at noob.com>sub   2048R/90123456 2010-01-01sub   2048R/78901234 2012-08-27
$ gpg --export --armor 78901234 > 78901234.txt
$ gpg --export --armor 12345678 > 12345678.txt
$ diff 78901234.txt 12345678.txt
Why is there no difference between the subkey and my public key ?

Maybe I misunderstood something...
 		 	   		  
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