SORRY.
After the Battle of Blackwater (which Robb and the gang weren't in, but just using as a time reference) Catelyn finds out that Robb married Jeyne (now it's Talisa and I can't fucking remember if they married last season, but I guess they did?), and she is, of course, pissed because she realizes the Frey's will be super pissed. So they have to figure out how to make the Frey's not so pissed, which is where Edmure's wedding comes into it, and leads to the RW.
The men from Harrenhal do join up with Robb prior to the RW, so I guess it really doesn't matter that Robb went to Harrenhal on the show. I really think they are just stretching that storyline out a bit so that Robb will be on the show more this season.
After the Battle of Blackwater (which Robb and the gang weren't in, but just using as a time reference) Catelyn finds out that Robb married Jeyne (now it's Talisa and I can't fucking remember if they married last season, but I guess they did?), and she is, of course, pissed because she realizes the Frey's will be super pissed. So they have to figure out how to make the Frey's not so pissed, which is where Edmure's wedding comes into it, and leads to the RW.
The men from Harrenhal do join up with Robb prior to the RW, so I guess it really doesn't matter that Robb went to Harrenhal on the show. I really think they are just stretching that storyline out a bit so that Robb will be on the show more this season.