I think it's an LJ problem. Looking at the DW-generated RSS and what shows up on the LJ source, it looks like the part of the code that in DW decides which icon to show (DW or LJ username) is confusing whatever does the Livejournal rendering. Probably because, as you say, the usernames are not the same but the LJ rendering assumes they are! Oh dear...
Re: Comment made on LJ syndicated blog but copied here in case you don't spot it on LJ
Date: 2013-06-04 05:14 pm (UTC)